Make first letter of a string upper case (with maximum performance)

I have a DetailsView with a TextBox and I want the input data be saved always with the first letter in capital.

Example:

"red" --> "Red"
"red house" --> " Red house"

How can I achieve this maximizing performance?

---

Note:

Based on the answers and the comments under the answers, many people think this is asking about capitalizing all words in the string. E.g. => Red House It isn't, but if that is what you seek, look for one of the answers that uses TextInfo's ToTitleCase method. (Note: Those answers are incorrect for the question actually asked.) See TextInfo.ToTitleCase documentation for caveats (doesn't touch all-caps words - they are considered acronyms; may lowercase letters in middle of words that "shouldn't" be lowered, e.g., "McDonald" → "Mcdonald"; not guaranteed to handle all culture-specific subtleties re capitalization rules.)

---

Note:

The question is ambiguous as to whether letters after the first should be forced to lower case. The accepted answer assumes that only the first letter should be altered. If you want to force all letters in the string except the first to be lower case, look for an answer containing ToLower, and not containing ToTitleCase.

Solution in different C# versions

C# 8 with at least .NET Core 3.0 or .NET Standard 2.1

public static class StringExtensions
{
    public static string FirstCharToUpper(this string input) =>
        input switch
        {
            null => throw new ArgumentNullException(nameof(input)),
            "" => throw new ArgumentException($"{nameof(input)} cannot be empty", nameof(input)),
            _ => string.Concat(input[0].ToString().ToUpper(), input.AsSpan(1))
        };
}

Since .NET Core 3.0 / .NET Standard 2.1 String.Concat() supports ReadonlySpan<char> which saves one allocation if we use .AsSpan(1) instead of .Substring(1).

C# 8

public static class StringExtensions
{
    public static string FirstCharToUpper(this string input) =>
        input switch
        {
            null => throw new ArgumentNullException(nameof(input)),
            "" => throw new ArgumentException($"{nameof(input)} cannot be empty", nameof(input)),
            _ => input[0].ToString().ToUpper() + input.Substring(1)
        };
}

C# 7

public static class StringExtensions
{
    public static string FirstCharToUpper(this string input)
    {
        switch (input)
        {
            case null: throw new ArgumentNullException(nameof(input));
            case "": throw new ArgumentException($"{nameof(input)} cannot be empty", nameof(input));
            default: return input[0].ToString().ToUpper() + input.Substring(1);
        }
    }
}

Really old answers

public static string FirstCharToUpper(string input)
{
    if (String.IsNullOrEmpty(input))
        throw new ArgumentException("ARGH!");
    return input.First().ToString().ToUpper() + String.Join("", input.Skip(1));
}

This version is shorter. For a faster solution, take a look at Diego's answer.

public static string FirstCharToUpper(string input)
{
    if (String.IsNullOrEmpty(input))
        throw new ArgumentException("ARGH!");
    return input.First().ToString().ToUpper() + input.Substring(1);
}

Probably the fastest solution is Darren's (There's even a benchmark) although I would change it's string.IsNullOrEmpty(s) validation to throw an exception since the original requirement expects for a first letter to exist so it can be uppercased. Note that this code works for a generic string and not particularly on valid values from the Textbox.

public string FirstLetterToUpper(string str)
{
    if (str == null)
        return null;

    if (str.Length > 1)
        return char.ToUpper(str[0]) + str.Substring(1);

    return str.ToUpper();
}

Old answer: This makes every first letter to upper case

public string ToTitleCase(string str)
{
	return CultureInfo.CurrentCulture.TextInfo.ToTitleCase(str.ToLower());
}

The right way is to use Culture:

System.Globalization.CultureInfo.CurrentCulture.TextInfo.ToTitleCase(word.ToLower())

Note: This will capitalise each word within a string, e.g. "red house" --> "Red House". The solution will also lower-case capitalisation within words, e.g. "old McDonald" --> "Old Mcdonald".

I took the fastest method from C# Uppercase First Letter - Dot Net Perls and converted to an extension method:

    /// <summary>
    /// Returns the input string with the first character converted to uppercase, or mutates any nulls passed into string.Empty
    /// </summary>
    public static string FirstLetterToUpperCaseOrConvertNullToEmptyString(this string s)
    {
        if (string.IsNullOrEmpty(s))
            return string.Empty;

        char[] a = s.ToCharArray();
        a[0] = char.ToUpper(a[0]);
        return new string(a);
    }

NOTE: The reason using ToCharArray is faster than the alternative char.ToUpper(s[0]) + s.Substring(1), is that only one string is allocated, whereas the Substring approach allocates a string for the substring, and then a second string to compose the final result.

---

Here is what this approach looks like, combined with the initial test from CarlosMuñoz's accepted answer:

    /// <summary>
    /// Returns the input string with the first character converted to uppercase
    /// </summary>
    public static string FirstLetterToUpperCase(this string s)
    {
        if (string.IsNullOrEmpty(s))
            throw new ArgumentException("There is no first letter");

        char[] a = s.ToCharArray();
        a[0] = char.ToUpper(a[0]);
        return new string(a);
    }

You can use the "ToTitleCase method":

string s = new CultureInfo("en-US").TextInfo.ToTitleCase("red house");
//result : Red House

This extension method solves every titlecase problem.

It is easy to use:

string str = "red house";
str.ToTitleCase();
//result : Red house

string str = "red house";
str.ToTitleCase(TitleCase.All);
//result : Red House

The extension method:

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Globalization;

namespace Test
{
    public static class StringHelper
    {
        private static CultureInfo ci = new CultureInfo("en-US");
        //Convert all first latter
        public static string ToTitleCase(this string str)
        {
            str = str.ToLower();
            var strArray = str.Split(' ');
            if (strArray.Length > 1)
            {
                strArray[0] = ci.TextInfo.ToTitleCase(strArray[0]);
                return string.Join(" ", strArray);
            }
            return ci.TextInfo.ToTitleCase(str);
        }

        public static string ToTitleCase(this string str, TitleCase tcase)
        {
            str = str.ToLower();
            switch (tcase)
            {
                case TitleCase.First:
                    var strArray = str.Split(' ');
                    if (strArray.Length > 1)
                    {
                        strArray[0] = ci.TextInfo.ToTitleCase(strArray[0]);
                        return string.Join(" ", strArray);
                    }
                    break;
                case TitleCase.All:
                    return ci.TextInfo.ToTitleCase(str);
                default:
                    break;
            }
            return ci.TextInfo.ToTitleCase(str);
        }
    }

    public enum TitleCase
    {
        First,
        All
    }
}

For the first letter, with error checking:

public string CapitalizeFirstLetter(string s)
{
    if (String.IsNullOrEmpty(s))
        return s;
    if (s.Length == 1)
        return s.ToUpper();
    return s.Remove(1).ToUpper() + s.Substring(1);
}

And here's the same as a handy extension

public static string CapitalizeFirstLetter(this string s)
{
    if (String.IsNullOrEmpty(s))
        return s;
    if (s.Length == 1)
        return s.ToUpper();
    return s.Remove(1).ToUpper() + s.Substring(1);
}

public static string ToInvarianTitleCase(this string self)
{
    if (string.IsNullOrWhiteSpace(self))
    {
        return self;
    }

    return CultureInfo.InvariantCulture.TextInfo.ToTitleCase(self);
}

Using string.Create() and avoiding the throw keyword in our method (yes, you read it right), we can take Marcell's answer one step further. Also, my method handles strings of arbitrary length (e.g., several megabytes of text).

public static string L33t(this string s)
{
    static void ThrowError() => throw new ArgumentException("There is no first letter");

    if (string.IsNullOrEmpty(s))
        ThrowError();                      // No "throw" keyword to avoid costly IL

    return string.Create(s.Length, s, (chars, state) =>
    {
        state.AsSpan().CopyTo(chars);      // No slicing to save some CPU cycles
        chars[0] = char.ToUpper(chars[0]);
    });
}

Performance

Here are the numbers for benchmarks run on .NET Core 3.1.7, 64 bit. I added a longer string to pinpoint the cost of extra copies.

Method	Data	Mean	Error	StdDev	Median
L33t	red	8.545 ns	0.4612 ns	1.3308 ns	8.075 ns
Marcell	red	9.153 ns	0.3377 ns	0.9471 ns	8.946 ns
L33t	red house	7.715 ns	0.1741 ns	0.4618 ns	7.793 ns
Marcell	red house	10.537 ns	0.5002 ns	1.4351 ns	10.377 ns
L33t	red r(...)house [89]	11.121 ns	0.6774 ns	1.9106 ns	10.612 ns
Marcell	red r(...)house [89]	16.739 ns	0.4468 ns	1.3033 ns	16.853 ns

Full test code

using System;
using BenchmarkDotNet.Attributes;
using BenchmarkDotNet.Running;

namespace CorePerformanceTest
{
    class Program
    {
        static void Main(string[] args)
        {
            var summary = BenchmarkRunner.Run<StringUpperTest>();
        }
    }

    public class StringUpperTest
    {
        [Params("red", "red house", "red red red red red red red red red red red red red red red red red red red red red house")]
        public string Data;

        [Benchmark]
        public string Marcell() => Data.Marcell();

        [Benchmark]
        public string L33t() => Data.L33t();
    }

    internal static class StringExtensions
    {
        public static string Marcell(this string s)
        {
            if (string.IsNullOrEmpty(s))
                throw new ArgumentException("There is no first letter");

            Span<char> a = stackalloc char[s.Length];
            s.AsSpan(1).CopyTo(a.Slice(1));
            a[0] = char.ToUpper(s[0]);
            return new string(a);
        }

        public static string L33t(this string s)
        {
            static void ThrowError() => throw new ArgumentException("There is no first letter");

            if (string.IsNullOrEmpty(s))
                ThrowError(); // IMPORTANT: Do not "throw" here!

            return string.Create(s.Length, s, (chars, state) =>
            {
                state.AsSpan().CopyTo(chars);
                chars[0] = char.ToUpper(chars[0]);
            });
        }
    }
}

Please let me know if you can make it any faster!

The fastest method:

private string Capitalize(string s){
    if (string.IsNullOrEmpty(s))
    {
        return string.Empty;
    }
    char[] a = s.ToCharArray();
    a[0] = char.ToUpper(a[0]);
    return new string(a);
}

Tests show the next results (string with 1,0000,000 symbols as input):

Test results

As this question is about maximizing performance, I adopted Darren's version to use Spans, which reduces garbage and improves speed by about 10%.

/// <summary>
/// Returns the input string with the first character converted to uppercase
/// </summary>
public static string ToUpperFirst(this string s)
{
    if (string.IsNullOrEmpty(s))
        throw new ArgumentException("There is no first letter");

    Span<char> a = stackalloc char[s.Length];
    s.AsSpan(1).CopyTo(a.Slice(1));
    a[0] = char.ToUpper(s[0]);
    return new string(a);
}

Performance

Method	Data	Mean	Error	StdDev
Carlos	red	107.29 ns	2.2401 ns	3.9234 ns
Darren	red	30.93 ns	0.9228 ns	0.8632 ns
Marcell	red	26.99 ns	0.3902 ns	0.3459 ns
Carlos	red house	106.78 ns	1.9713 ns	1.8439 ns
Darren	red house	32.49 ns	0.4253 ns	0.3978 ns
Marcell	red house	27.37 ns	0.3888 ns	0.3637 ns

Full test code

using System;
using System.Linq;

using BenchmarkDotNet.Attributes;

namespace CorePerformanceTest
{
    public class StringUpperTest
    {
        [Params("red", "red house")]
        public string Data;

        [Benchmark]
        public string Carlos() => Data.Carlos();

        [Benchmark]
        public string Darren() => Data.Darren();

        [Benchmark]
        public string Marcell() => Data.Marcell();
    }

    internal static class StringExtensions
    {
        public static string Carlos(this string input) =>
            input switch
            {
                null => throw new ArgumentNullException(nameof(input)),
                "" => throw new ArgumentException($"{nameof(input)} cannot be empty", nameof(input)),
                _ => input.First().ToString().ToUpper() + input.Substring(1)
            };

        public static string Darren(this string s)
        {
            if (string.IsNullOrEmpty(s))
                throw new ArgumentException("There is no first letter");

            char[] a = s.ToCharArray();
            a[0] = char.ToUpper(a[0]);
            return new string(a);
        }

        public static string Marcell(this string s)
        {
            if (string.IsNullOrEmpty(s))
                throw new ArgumentException("There is no first letter");

            Span<char> a = stackalloc char[s.Length];
            s.AsSpan(1).CopyTo(a.Slice(1));
            a[0] = char.ToUpper(s[0]);
            return new string(a);
        }
    }

}

Try this:

static public string UpperCaseFirstCharacter(this string text) {
    return Regex.Replace(text, "^[a-z]", m => m.Value.ToUpper());
}

Check if the string is not null, convert the first character to upper case and the rest of them to lower case:

public static string FirstCharToUpper(string str)
{
    return str?.First().ToString().ToUpper() + str?.Substring(1).ToLower();
}

If performance/memory usage is an issue then, this one only creates one (1) StringBuilder and one (1) new String of the same size as the original string.

public static string ToUpperFirst(this string str) {
  if(!string.IsNullOrEmpty(str)) {
    StringBuilder sb = new StringBuilder(str);
    sb[0] = char.ToUpper(sb[0]);

    return sb.ToString();

  } else return str;
}

Since I happened to be working on this also, and was looking around for any ideas, this is the solution I came to. It uses LINQ, and will be able to capitalize the first letter of a string, even if the first occurrence isn't a letter. Here's the extension method I ended up making.

public static string CaptalizeFirstLetter(this string data)
{
    var chars = data.ToCharArray();
        
    // Find the Index of the first letter
    var charac = data.First(char.IsLetter);
    var i = data.IndexOf(charac);
        
    // capitalize that letter
    chars[i] = char.ToUpper(chars[i]);

    return new string(chars);
}

I'm sure there's a way to optimize or clean this up a little bit.

If you only care about the first letter being capitalized and it does not matter the rest of the string, you can just select the first character, make it upper case and concatenate it with the rest of the string without the original first character.

String word ="red house";
word = word[0].ToString().ToUpper() + word.Substring(1, word.length -1);
//result: word = "Red house"

We need to convert the first character ToString(), because we are reading it as a Char array, and the Char type does not have a ToUpper() method.

Here's a way to do it as an extension method:

static public string UpperCaseFirstCharacter(this string text)
{
    if (!string.IsNullOrEmpty(text))
    {
        return string.Format(
            "{0}{1}",
            text.Substring(0, 1).ToUpper(),
            text.Substring(1));
    }

    return text;
}

It can then be called like:

//yields "This is Brian's test.":
"this is Brian's test.".UpperCaseFirstCharacter();

And here are some unit tests for it:

[Test]
public void UpperCaseFirstCharacter_ZeroLength_ReturnsOriginal()
{
    string orig = "";
    string result = orig.UpperCaseFirstCharacter();

    Assert.AreEqual(orig, result);
}

[Test]
public void UpperCaseFirstCharacter_SingleCharacter_ReturnsCapital()
{
    string orig = "c";
    string result = orig.UpperCaseFirstCharacter();

    Assert.AreEqual("C", result);
}

[Test]
public void UpperCaseFirstCharacter_StandardInput_CapitalizeOnlyFirstLetter()
{
    string orig = "this is Brian's test.";
    string result = orig.UpperCaseFirstCharacter();

    Assert.AreEqual("This is Brian's test.", result);
}

I found something in C# Uppercase First Letter - Dot Net Perls:

static string UppercaseFirst(string s)
{
    // Check for empty string.
    if (string.IsNullOrEmpty(s))
    {
        return string.Empty;
    }

    // Return char and concat substring.
    return char.ToUpper(s[0]) + s.Substring(1);
}

This will do it although it will also make sure that there are no errant capitals that are not at the beginning of the word.

public string(string s)
{
    System.Globalization.CultureInfo c = new System.Globalization.CultureInfo("en-us", false)
    System.Globalization.TextInfo t = c.TextInfo;

    return t.ToTitleCase(s);
}

string emp="TENDULKAR";
string output;
output=emp.First().ToString().ToUpper() + String.Join("", emp.Skip(1)).ToLower();

There seems to be a lot of complexity here when all you need is:

/// <summary>
/// Returns the input string with the first character converted to uppercase if a letter
/// </summary>
/// <remarks>Null input returns null</remarks>
public static string FirstLetterToUpperCase(this string s)
{
    if (string.IsNullOrWhiteSpace(s))
        return s;

    return char.ToUpper(s[0]) + s.Substring(1);
}

Noteworthy points:

1. It's an extension method.

2. If the input is null, empty or white space the input is returned as is.

3. String.IsNullOrWhiteSpace was introduced with .NET Framework 4. This won't work with older frameworks.

I wanted to provide a "maximum performance" answer. In my mind, a "maximum performance" answer catches all scenarios and provides the answer to the question accounting for those scenarios. So, here is my answer. With these reasons:

1. IsNullOrWhiteSpace accounts for strings that are just spaces or null/empty.

2. .Trim() removes white space from the front and back of the string.

3. .First() takes the first element of an IEnumerable<TSource> (or string).

4. We should check to see if it is a letter that can/should be uppercase.

5. We then add the rest of the string, only if the length indicates we should.

6. By .NET best practice, we should provide a culture under System.Globalization.CultureInfo.

7. Providing them as optional parameters makes this method totally reusable, without having to type the chosen culture every time.

8. I also noticed that my and most of these answers did not maintain the whitespace at the beginning of the string. This will also show how to maintain that whitespace.

   //Capitalize the first letter disregard all chars using regex.
   public static string RegCapString(this string instring, string culture = "en-US", bool useSystem = false)
   {
       if (string.IsNullOrWhiteSpace(instring))
       {
           return instring;
       }
       var m = Regex.Match(instring, "[A-Za-z]").Index;
       return instring.Substring(0, m) + instring[m].ToString().ToUpper(new CultureInfo(culture, useSystem)) + instring.Substring(m + 1);
   }
   //Capitalize first char if it is a letter disregard white space.
   public static string CapString(this string instring, string culture = "en-US", bool useSystem = false)
   {
       if (string.IsNullOrWhiteSpace(instring) ||
          !char.IsLetter(instring.Trim().First()))
       {
           return instring;
       }
       var whiteSpaces = instring.Length - instring.TrimStart().Length;
       return (new string(' ', whiteSpaces)) + 
           instring.Trim().First().ToString().ToUpper(new CultureInfo(culture, useSystem)) + 
           ((instring.TrimStart().Length > 1) ? instring.Substring(whiteSpaces + 1) : "");
   }
   

I think the below method is the best solution.

class Program
{
    static string UppercaseWords(string value)
    {
        char[] array = value.ToCharArray();
        // Handle the first letter in the string.
        if (array.Length >= 1)
        {
            if (char.IsLower(array[0]))
            {
                array[0] = char.ToUpper(array[0]);
            }
        }
        // Scan through the letters, checking for spaces.
        // ... Uppercase the lowercase letters following spaces.
        for (int i = 1; i < array.Length; i++)
        {
            if (array[i - 1] == ' ')
            {
                if (char.IsLower(array[i]))
                {
                    array[i] = char.ToUpper(array[i]);
                }
            }
        }
        return new string(array);
    }

    static void Main()
    {
        // Uppercase words in these strings.
        const string value1 = "something in the way";
        const string value2 = "dot net PERLS";
        const string value3 = "String_two;three";
        const string value4 = " sam";
        // ... Compute the uppercase strings.
        Console.WriteLine(UppercaseWords(value1));
        Console.WriteLine(UppercaseWords(value2));
        Console.WriteLine(UppercaseWords(value3));
        Console.WriteLine(UppercaseWords(value4));
    }
}

Output

Something In The Way
Dot Net PERLS
String_two;three
 Sam

Reference

Possible solution to resolve your problem:

   public static string FirstToUpper(this string lowerWord)
   {
       if (string.IsNullOrWhiteSpace(lowerWord) || string.IsNullOrEmpty(lowerWord))
            return lowerWord;
       return new StringBuilder(lowerWord.Substring(0, 1).ToUpper())
                 .Append(lowerWord.Substring(1))
                 .ToString();
   }

We can do this (C# 8.0, .NET 5):

input?.Length > 0 ? char.ToUpperInvariant(input[0]) + input[1..] : input

I believe that is short enough to do inline.

---

If input is an empty string, we get an empty string. If input is null, we get null.

Otherwise, the code takes the first character input[0] and convert it to uppercase with char.ToUpperInvariant. And concatenate the rest input[1..].

The compiler will convert the range access to a call to Substring, plus it can take advantage of the fact that we already got the length.

Over the accepted answer, this has the advantage of not using LINQ. Some other answers convert the string to array just to take the first character. This code does not do that either.

---

If you prefer an extension method, you can do this:

public static string FirstCharToUpper(this string input) =>
        input?.Length > 0 ? char.ToUpperInvariant(input[0]) + input[1..] : input;

What if you prefer to throw? OK, have it throw:

public static string FirstCharToUpper(this string input) =>
        input switch
        {
            null => throw new ArgumentNullException(nameof(input)),
            _ => input.Length > 0 ? char.ToUpperInvariant(input[0]) + input[1..] : input
        };

Here roughly equivalent code (since we are making an extension method, we can be a bit more verbose):

public static string FirstCharToUpperEquivalent(this string input)
{
    if (input == null)
    {
        throw new ArgumentNullException(nameof(input));
    }

    var length = input.Length;
    if (length == 0)
    {
        return input;
    }

    string firstCharacter = char.ToUpperInvariant(input[0]).ToString();
    return string.Concat(firstCharacter, input.Substring(1, length - 1));
}

---

I did a benchmark with 1000 rounds of 155 words (So they were called 155000 times), and this is the result:

Benchmarking type Tests
  TestAccepted         00:00:00.0465979
  TestProposalNoThrow  00:00:00.0092839
  TestProposalDoThrow  00:00:00.0092938
  TestProposalEquival  00:00:00.0091463

I ran it on Windows 10, Intel Core i3, using the code from Simple microbenchmarking in C# by Jon Skeet.

It seems like none of the solutions given here will deal with a white space before the string.

Just adding this as a thought:

public static string SetFirstCharUpper2(string aValue, bool aIgonreLeadingSpaces = true)
{
    if (string.IsNullOrWhiteSpace(aValue))
        return aValue;

    string trimmed = aIgonreLeadingSpaces
           ? aValue.TrimStart()
           : aValue;

    return char.ToUpper(trimmed[0]) + trimmed.Substring(1);
}

It should handle this won't work on other answers (that sentence has a space in the beginning), and if you don't like the space trimming, just pass a false as the second parameter (or change the default to false, and pass true if you want to deal with space)).

FluentSharp has the lowerCaseFirstLetter method which does this.

This is the fastest way:

public static unsafe void ToUpperFirst(this string str)
{
    if (str == null)
        return;
    fixed (char* ptr = str)
        *ptr = char.ToUpper(*ptr);
}

Without changing the original string:

public static unsafe string ToUpperFirst(this string str)
{
    if (str == null)
        return null;
    string ret = string.Copy(str);
    fixed (char* ptr = ret)
        *ptr = char.ToUpper(*ptr);
    return ret;
}

The following function is correct for all ways:

static string UppercaseWords(string value)
{
    char[] array = value.ToCharArray();
    // Handle the first letter in the string.
    if (array.Length >= 1)
    {
        if (char.IsLower(array[0]))
        {
            array[0] = char.ToUpper(array[0]);
        }
    }
    // Scan through the letters, checking for spaces.
    // ... Uppercase the lowercase letters following spaces.
    for (int i = 1; i < array.Length; i++)
    {
        if (array[i - 1] == ' ')
        {
            if (char.IsLower(array[i]))
            {
                array[i] = char.ToUpper(array[i]);
            }
        }
    }
    return new string(array);
}

I found that here.

Expanding on Carlos' question above, if you want to capitalize multiple sentences you may use this code:

    /// <summary>
    /// Capitalize first letter of every sentence. 
    /// </summary>
    /// <param name="inputSting"></param>
    /// <returns></returns>
    public string CapitalizeSentences (string inputSting)
    {
        string result = string.Empty;
        if (!string.IsNullOrEmpty(inputSting))
        {
            string[] sentences = inputSting.Split('.');

            foreach (string sentence in sentences)
            {
                result += string.Format ("{0}{1}.", sentence.First().ToString().ToUpper(), sentence.Substring(1)); 
            }
        }

        return result; 
    }

Recently I had a similar requirement and remembered that the LINQ function Select() provides an index:

string input;
string output;

input = "red house";
output = String.Concat(input.Select((currentChar, index) => index == 0 ? Char.ToUpper(currentChar) : currentChar));
//output = "Red house"

Since I need that very often I made an extension method for the string type:

public static class StringExtensions
{
    public static string FirstLetterToUpper(this string input)
    {
        if (string.IsNullOrEmpty(input))
            return string.Empty;
        return String.Concat(input.Select((currentChar, index) => index == 0 ? Char.ToUpper(currentChar) : currentChar));
    }
}

Please note that only the first letter is converted to upper case - all remaining characters are not touched. If you need the other characters to be lower case you may also call Char.ToLower(currentChar) for index > 0 or call ToLower() on the whole string in the first place.

Regarding performance I compared the code with the solution from Darren. On my machine Darren's code is about two times faster which is no surprise since he's directly editing only the first letter within a char array.

So I suggest you take Darren's code if you need the fastest solution available. If you want to integrate other string manipulations as well it may be convenient to have the expressive power of a lambda function touching the characters of the input string - you can easily extend this function - so I leave this solution here.

Here's the code that worked for me:

private string StringLetterUppercase(string input)
{
    if (input == null)
    {
        throw new ArgumentNullException(nameof(input));
    }
    else if (input == "")
    {
        throw new ArgumentException($"{nameof(input)} cannot be empty", nameof(input));
    }
    else
    {
        return input.First().ToString().ToUpper() + input.Substring(1);
    }
}

This capitalizes the first letter and every letter following a space and lower cases any other letter.

public string CapitalizeFirstLetterAfterSpace(string input)
{
    System.Text.StringBuilder sb = new System.Text.StringBuilder(input);
    bool capitalizeNextLetter = true;
    for(int pos = 0; pos < sb.Length; pos++)
    {
        if(capitalizeNextLetter)
        {
            sb[pos]=System.Char.ToUpper(sb[pos]);
            capitalizeNextLetter = false;
        }
        else
        {
            sb[pos]=System.Char.ToLower(sb[pos]);
        }

        if(sb[pos]=' ')
        {
            capitalizeNextLetter=true;
        }
    }
}

Use the following code:

string strtest ="PRASHANT";
strtest.First().ToString().ToUpper() + strtest.Remove(0, 1).ToLower();

The easiest way to capitalize the first letter is:

1. Using System.Globalization;

    // Creates a TextInfo based on the "en-US" culture.
    TextInfo myTI = new CultureInfo("en-US", false).
   
    myTI.ToTitleCase(textboxname.Text)
   

string input = "red HOUSE";
System.Text.StringBuilder sb = new System.Text.StringBuilder(input);

for (int j = 0; j < sb.Length; j++)
{
    if ( j == 0 ) //catches just the first letter
        sb[j] = System.Char.ToUpper(sb[j]);
    else  //everything else is lower case
        sb[j] = System.Char.ToLower(sb[j]);
}
// Store the new string.
string corrected = sb.ToString();
System.Console.WriteLine(corrected);

As BobBeechey suggests in his response to this question, the following code will work for this:

private void txt_fname_TextChanged(object sender, EventArgs e)
{
    char[] c = txt_fname.Text.ToCharArray();
    int j;
    for (j = 0; j < txt_fname.Text.Length; j++)
    {
        if (j==0) c[j]=c[j].ToString().ToUpper()[0];
        else c[j] = c[j].ToString().ToLower()[0];
    }
    txt_fname.Text = new string(c);	
    txt_fname.Select(txt_fname.Text.Length, 1);
}

With this method you can upper the first character of every word.

Example

"HeLlo wOrld" => "Hello World"

public static string FirstCharToUpper(string input)
{
    if (String.IsNullOrEmpty(input))
        throw new ArgumentException("Error");
    return string.Join(" ", input.Split(' ').Select(d => d.First().ToString().ToUpper() +  d.ToLower().Substring(1)));
}

I use this to correct names. It basically works on the concept of changing a character to uppercase if it follows a specific pattern. In this case I've gone for space and dash of "Mc".

private String CorrectName(String name)
{
    List<String> StringsToCapitalizeAfter = new List<String>() { " ", "-", "Mc" };
    StringBuilder NameBuilder = new StringBuilder();
    name.Select(c => c.ToString()).ToList().ForEach(c =>
    {
        c = c.ToLower();
        StringsToCapitalizeAfter.ForEach(s =>
        {
            if(String.IsNullOrEmpty(NameBuilder.ToString()) ||
               NameBuilder.ToString().EndsWith(s))
            {
                c = c.ToUpper();
            }
        });
        NameBuilder.Append(c);
    });
    return NameBuilder.ToString();
}

 private string capitalizeFirstCharacter(string format)
 {
     if (string.IsNullOrEmpty(format))
         return string.Empty;
     else
         return char.ToUpper(format[0]) + format.ToLower().Substring(1);
 }

string s_Val = "test"; if (s_Val != "") { s_Val = char.ToUpper(s_Val[0]); if (s_Val.Length > 1) { s_Val += s_Val.Substring(1); } }

The simplest and fastest way is to replace the first character of the string by making it an upper case character:

string str = "test";<br>
str = str.Replace(str[0], char.ToUpper(str[0]));