What's a good way of doing string templating in .NET?

I need to send email notifications to users and I need to allow the admin to provide a template for the message body (and possibly headers, too).

I'd like something like string.Format that allows me to give named replacement strings, so the template can look like this:

Dear {User},

Your job finished at {FinishTime} and your file is available for download at {FileURL}.

Regards,

-- 
{Signature}

What's the simplest way for me to do that?

Here is the version for those of you who can use a new version of C#:

// add $ at start to mark string as template
var template = $"Your job finished at {FinishTime} and your file is available for download at {FileURL}."

In a line - this is now a fully supported language feature (string interpolation).

Use a templating engine. StringTemplate is one of those, and there are many.

Example:

using Antlr.StringTemplate;
using Antlr.StringTemplate.Language;
 
StringTemplate hello = new StringTemplate("Hello, $name$", typeof(DefaultTemplateLexer));
hello.SetAttribute("name", "World");
Console.Out.WriteLine(hello.ToString());

You can use the "string.Format" method:

var user = GetUser();
var finishTime = GetFinishTime();
var fileUrl = GetFileUrl();
var signature = GetSignature();
string msg =
@"Dear {0},

Your job finished at {1} and your file is available for download at {2}.

Regards,

--
{3}";
msg = string.Format(msg, user, finishTime, fileUrl, signature);

It allows you to change the content in the future and is friendly for localization.

I wrote a pretty simple library, SmartFormat which meets all your requirements. It is focused on composing "natural language" text, and is great for generating data from lists, or applying conditional logic.

The syntax is extremely similar to String.Format, and is very simple and easy to learn and use. Here's an example of the syntax from the documentation:

Smart.Format("{Name}'s friends: {Friends:{Name}|, |, and}", user)
// Result: "Scott's friends: Michael, Jim, Pam, and Dwight"

The library has great error-handling options (ignore errors, output errors, throw errors). Obviously, this would work perfect for your example.

The library is open source and easily extensible, so you can also enhance it with additional features too.

Building on Benjamin Gruenbaum's answer, in C# version 6 you can add a @ with the $ and pretty much use your code as it is, e.g.:

var text = $@"Dear {User},

Your job finished at {FinishTime} and your file is available for download at {FileURL}.

Regards,

-- 
{Signature}
";

The $ is for string interpolation: https://docs.microsoft.com/en-us/dotnet/csharp/language-reference/tokens/interpolated

The @ is the verbatim identifier: https://docs.microsoft.com/en-us/dotnet/csharp/language-reference/tokens/verbatim

...and you can use these in conjunction.

:o)

You could use string.Replace(...), eventually in a for-each through all the keywords. If there are only a few keywords you can have them on a line like this:

string myString = template.Replace("FirstName", "John").Replace("LastName", "Smith").Replace("FinishTime", DateTime.Now.ToShortDateString());

Or you could use Regex.Replace(...), if you need something a bit more powerful and with more options.

Read this article on codeproject to view which string replacement option is fastest for you.

A very simple regex-based solution. Supports \n-style single character escape sequences and {Name}-style named variables.

Source

class Template
{
    /// <summary>Map of replacements for characters prefixed with a backward slash</summary>
    private static readonly Dictionary<char, string> EscapeChars
        = new Dictionary<char, string>
        {
            ['r'] = "\r",
            ['n'] = "\n",
            ['\\'] = "\\",
            ['{'] = "{",
        };

    /// <summary>Pre-compiled regular expression used during the rendering process</summary>
    private static readonly Regex RenderExpr = new Regex(@"\\.|{([a-z0-9_.\-]+)}",
        RegexOptions.IgnoreCase | RegexOptions.Compiled);

    /// <summary>Template string associated with the instance</summary>
    public string TemplateString { get; }

    /// <summary>Create a new instance with the specified template string</summary>
    /// <param name="TemplateString">Template string associated with the instance</param>
    public Template(string TemplateString)
    {
        if (TemplateString == null) {
            throw new ArgumentNullException(nameof(TemplateString));
        }

        this.TemplateString = TemplateString;
    }

    /// <summary>Render the template using the supplied variable values</summary>
    /// <param name="Variables">Variables that can be substituted in the template string</param>
    /// <returns>The rendered template string</returns>
    public string Render(Dictionary<string, object> Variables)
    {
        return Render(this.TemplateString, Variables);
    }

    /// <summary>Render the supplied template string using the supplied variable values</summary>
    /// <param name="TemplateString">The template string to render</param>
    /// <param name="Variables">Variables that can be substituted in the template string</param>
    /// <returns>The rendered template string</returns>
    public static string Render(string TemplateString, Dictionary<string, object> Variables)
    {
        if (TemplateString == null) {
            throw new ArgumentNullException(nameof(TemplateString));
        }

        return RenderExpr.Replace(TemplateString, Match => {
            switch (Match.Value[0]) {
                case '\\':
                    if (EscapeChars.ContainsKey(Match.Value[1])) {
                        return EscapeChars[Match.Value[1]];
                    }
                    break;

                case '{':
                    if (Variables.ContainsKey(Match.Groups[1].Value)) {
                        return Variables[Match.Groups[1].Value].ToString();
                    }
                    break;
            }

            return string.Empty;
        });
    }
}

Usage

var tplStr1 = @"Hello {Name},\nNice to meet you!";
var tplStr2 = @"This {Type} \{contains} \\ some things \\n that shouldn't be rendered";
var variableValues = new Dictionary<string, object>
{
    ["Name"] = "Bob",
    ["Type"] = "string",
};

Console.Write(Template.Render(tplStr1, variableValues));
// Hello Bob,
// Nice to meet you!

var template = new Template(tplStr2);
Console.Write(template.Render(variableValues));
// This string {contains} \ some things \n that shouldn't be rendered

Notes

• I've only defined \n, \r, \\ and \{ escape sequences and hard-coded them. You could easily add more or make them definable by the consumer.
• I've made variable names case-insensitive, as things like this are often presented to end-users/non-programmers and I don't personally think that case-sensitivity make sense in that use-case - it's just one more thing they can get wrong and phone you up to complain about (plus in general if you think you need case sensitive symbol names what you really need are better symbol names). To make them case-sensitive, simply remove the RegexOptions.IgnoreCase flag.
• I strip invalid variable names and escape sequences from the result string. To leave them intact, return Match.Value instead of the empty string at the end of the Regex.Replace callback. You could also throw an exception.
• I've used {var} syntax, but this may interfere with the native interpolated string syntax. If you want to define templates in string literals in you code, it might be advisable to change the variable delimiters to e.g. %var% (regex \\.|%([a-z0-9_.\-]+)%) or some other syntax of your choosing which is more appropriate to the use case.

Actually, you can use XSLT. You create a simple XML template:

<?xml version="1.0" encoding="utf-8"?>
<xsl:stylesheet version="1.0" xmlns:xsl="http://www.w3.org/1999/XSL/Transform" xmlns:msxsl="urn:schemas-microsoft-com:xslt" exclude-result-prefixes="msxsl">
  <xsl:template match="TETT">
    <p>
       Dear <xsl:variable name="USERNAME" select="XML_PATH" />,

       Your job finished at <xsl:variable name="FINISH_TIME" select="XML_PATH" /> and your file is available for download at <xsl:variable name="FILE_URL" select="XML_PATH" />.

       Regards,
        -- 
       <xsl:variable name="SIGNATURE" select="XML_PATH" />
    </p>
</xsl:template>

Then create a XmlDocument to perform transformation against: XmlDocument xmlDoc = new XmlDocument();

        XmlNode xmlNode = xmlDoc .CreateNode(XmlNodeType.Element, "EMAIL", null);
        XmlElement xmlElement= xmlDoc.CreateElement("USERNAME");
        xmlElement.InnerXml = username;
        xmlNode .AppendChild(xmlElement); ///repeat the same thing for all the required fields

        xmlDoc.AppendChild(xmlNode);

After that, apply the transformation:

        XPathNavigator xPathNavigator = xmlDocument.DocumentElement.CreateNavigator();
        StringBuilder sb = new StringBuilder();
        StringWriter sw = new StringWriter(sb);
        XmlTextWriter xmlWriter = new XmlTextWriter(sw);
        your_xslt_transformation.Transform(xPathNavigator, null, xmlWriter);
        return sb.ToString();

Implementing your own custom formatter might be a good idea.

Here's how you do it. First, create a type that defines the stuff you want to inject into your message. Note: I'm only going to illustrate this with the User part of your template...

class JobDetails
{
    public string User 
    { 
        get;
        set; 
    }        
}

Next, implement a simple custom formatter...

class ExampleFormatter : IFormatProvider, ICustomFormatter
{
    public object GetFormat(Type formatType)
    {
        return this;
    }

    public string Format(string format, object arg, IFormatProvider formatProvider)
    {
        // make this more robust
        JobDetails job = (JobDetails)arg;

        switch (format)
        {
            case "User":
            {
                return job.User;
            }
            default:
            {
                // this should be replaced with logic to cover the other formats you need
                return String.Empty;
            }
        }
    }
}

Finally, use it like this...

string template = "Dear {0:User}. Your job finished...";

JobDetails job = new JobDetails()
                     {
                             User = "Martin Peck"
                     };

string message = string.Format(new ExampleFormatter(), template, job);

... which will generate the text "Dear Martin Peck. Your job finished...".

In case someone is searching for an alternative -- an actual .NET one:

https://github.com/crozone/FormatWith | https://www.nuget.org/packages/FormatWith

A nice simple extendable solution. Thank you crozone!

So using the string extension provided in FormatWith here are two examples:

    static string emailTemplate = @"
Dear {User},

Your job finished at {FinishTime} and your file is available for download at {FileURL}.

-- 
{Signature}
    ";

//////////////////////////////////
/// 1. Use a dictionary that has the tokens as keys with values for the replacement
//////////////////////////////////
    public void TestUsingDictionary()
    {    
        var emailDictionary = new Dictionary<string, object>()
        {
            { "User", "Simon" },
            { "FinishTime", DateTime.Now },
            { "FileUrl", new Uri("http://example.com/dictionary") },
            { "Signature", $"Sincerely,{Environment.NewLine}Admin" }
        };

        var emailBody = emailTemplate.FormatWith(emailDictionary);

        System.Console.WriteLine(emailBody);
    }

//////////////////////////////////
/// 2. Use a poco with properties that match the replacement tokens
//////////////////////////////////
    public class MessageValues
    {
        public string User { get; set; } = "Simon";
        public DateTime FinishTime { get; set; } = DateTime.Now;
        public Uri FileURL { get; set; } = new Uri("http://example.com");
        public string Signature { get; set; } = $"Sincerely,{Environment.NewLine}Admin";
    }

    public void TestUsingPoco()
    {
        var emailBody = emailTemplate.FormatWith(new MessageValues());

        System.Console.WriteLine(emailBody);
    }

It allows formatting the replacement inline as well. For example, try changing {FinishTime} to {FinishTime:HH:mm:ss} in emailTemplate.

If you need something very powerful (but really not the simplest way) you can host ASP.NET and use it as your templating engine.

You'll have all the power of ASP.NET to format the body of your message.

If you are coding in VB.NET you can use XML literals. If you are coding in C# you can use ShartDevelop to have files in VB.NET in the same project as C# code.