Get the property, as a string, from an Expression<Func<TModel,TProperty>>
C#LambdaC# Problem Overview
I use some strongly-typed expressions that get serialized to allow my UI code to have strongly-typed sorting and searching expressions. These are of type Expression<Func<TModel,TProperty>> and are used as such: SortOption.Field = (p => p.FirstName);. I've gotten this working perfectly for this simple case.
The code that I'm using for parsing the "FirstName" property out of there is actually reusing some existing functionality in a third-party product that we use and it works great, until we start working with deeply-nested properties(SortOption.Field = (p => p.Address.State.Abbreviation);). This code has some very different assumptions in the need to support deeply-nested properties.
As for what this code does, I don't really understand it and rather than changing that code, I figured I should just write from scratch this functionality. However, I don't know of a good way to do this. I suspect we can do something better than doing a ToString() and performing string parsing. So what's a good way to do this to handle the trivial and deeply-nested cases?
Requirements:
- Given the expression
p => p.FirstNameI need a string of"FirstName". - Given the expression
p => p.Address.State.AbbreviationI need a string of"Address.State.Abbreviation"
While it's not important for an answer to my question, I suspect my serialization/deserialization code could be useful to somebody else who finds this question in the future, so it is below. Again, this code is not important to the question - I just thought it might help somebody. Note that DynamicExpression.ParseLambda comes from the Dynamic LINQ stuff and Property.PropertyToString() is what this question is about.
/// <summary>
/// This defines a framework to pass, across serialized tiers, sorting logic to be performed.
/// </summary>
/// <typeparam name="TModel">This is the object type that you are filtering.</typeparam>
/// <typeparam name="TProperty">This is the property on the object that you are filtering.</typeparam>
[Serializable]
public class SortOption<TModel, TProperty> : ISerializable where TModel : class
{
/// <summary>
/// Convenience constructor.
/// </summary>
/// <param name="property">The property to sort.</param>
/// <param name="isAscending">Indicates if the sorting should be ascending or descending</param>
/// <param name="priority">Indicates the sorting priority where 0 is a higher priority than 10.</param>
public SortOption(Expression<Func<TModel, TProperty>> property, bool isAscending = true, int priority = 0)
{
Property = property;
IsAscending = isAscending;
Priority = priority;
}
/// <summary>
/// Default Constructor.
/// </summary>
public SortOption()
: this(null)
{
}
/// <summary>
/// This is the field on the object to filter.
/// </summary>
public Expression<Func<TModel, TProperty>> Property { get; set; }
/// <summary>
/// This indicates if the sorting should be ascending or descending.
/// </summary>
public bool IsAscending { get; set; }
/// <summary>
/// This indicates the sorting priority where 0 is a higher priority than 10.
/// </summary>
public int Priority { get; set; }
#region Implementation of ISerializable
/// <summary>
/// This is the constructor called when deserializing a SortOption.
/// </summary>
protected SortOption(SerializationInfo info, StreamingContext context)
{
IsAscending = info.GetBoolean("IsAscending");
Priority = info.GetInt32("Priority");
// We just persisted this by the PropertyName. So let's rebuild the Lambda Expression from that.
Property = DynamicExpression.ParseLambda<TModel, TProperty>(info.GetString("Property"), default(TModel), default(TProperty));
}
/// <summary>
/// Populates a <see cref="T:System.Runtime.Serialization.SerializationInfo"/> with the data needed to serialize the target object.
/// </summary>
/// <param name="info">The <see cref="T:System.Runtime.Serialization.SerializationInfo"/> to populate with data. </param>
/// <param name="context">The destination (see <see cref="T:System.Runtime.Serialization.StreamingContext"/>) for this serialization. </param>
public void GetObjectData(SerializationInfo info, StreamingContext context)
{
// Just stick the property name in there. We'll rebuild the expression based on that on the other end.
info.AddValue("Property", Property.PropertyToString());
info.AddValue("IsAscending", IsAscending);
info.AddValue("Priority", Priority);
}
#endregion
}
C# Solutions
Solution 1 - C#
Here's the trick: any expression of this form...
obj => obj.A.B.C // etc.
...is really just a bunch of nested MemberExpression objects.
First you've got:
MemberExpression: obj.A.B.C
Expression: obj.A.B // MemberExpression
Member: C
Evaluating Expression above as a MemberExpression gives you:
MemberExpression: obj.A.B
Expression: obj.A // MemberExpression
Member: B
Finally, above that (at the "top") you have:
MemberExpression: obj.A
Expression: obj // note: not a MemberExpression
Member: A
So it seems clear that the way to approach this problem is by checking the Expression property of a MemberExpression up until the point where it is no longer itself a MemberExpression.
UPDATE: It seems there is an added spin on your problem. It may be that you have some lambda that looks like a Func<T, int>...
p => p.Age
...but is actually a Func<T, object>; in this case, the compiler will convert the above expression to:
p => Convert(p.Age)
Adjusting for this issue actually isn't as tough as it might seem. Take a look at my updated code for one way to deal with it. Notice that by abstracting the code for getting a MemberExpression away into its own method (TryFindMemberExpression), this approach keeps the GetFullPropertyName method fairly clean and allows you to add additional checks in the future -- if, perhaps, you find yourself facing a new scenario which you hadn't originally accounted for -- without having to wade through too much code.
To illustrate: this code worked for me.
// code adjusted to prevent horizontal overflow
static string GetFullPropertyName<T, TProperty>
(Expression<Func<T, TProperty>> exp)
{
MemberExpression memberExp;
if (!TryFindMemberExpression(exp.Body, out memberExp))
return string.Empty;
var memberNames = new Stack<string>();
do
{
memberNames.Push(memberExp.Member.Name);
}
while (TryFindMemberExpression(memberExp.Expression, out memberExp));
return string.Join(".", memberNames.ToArray());
}
// code adjusted to prevent horizontal overflow
private static bool TryFindMemberExpression
(Expression exp, out MemberExpression memberExp)
{
memberExp = exp as MemberExpression;
if (memberExp != null)
{
// heyo! that was easy enough
return true;
}
// if the compiler created an automatic conversion,
// it'll look something like...
// obj => Convert(obj.Property) [e.g., int -> object]
// OR:
// obj => ConvertChecked(obj.Property) [e.g., int -> long]
// ...which are the cases checked in IsConversion
if (IsConversion(exp) && exp is UnaryExpression)
{
memberExp = ((UnaryExpression)exp).Operand as MemberExpression;
if (memberExp != null)
{
return true;
}
}
return false;
}
private static bool IsConversion(Expression exp)
{
return (
exp.NodeType == ExpressionType.Convert ||
exp.NodeType == ExpressionType.ConvertChecked
);
}
Usage:
Expression<Func<Person, string>> simpleExp = p => p.FirstName;
Expression<Func<Person, string>> complexExp = p => p.Address.State.Abbreviation;
Expression<Func<Person, object>> ageExp = p => p.Age;
Console.WriteLine(GetFullPropertyName(simpleExp));
Console.WriteLine(GetFullPropertyName(complexExp));
Console.WriteLine(GetFullPropertyName(ageExp));
Output:
FirstName
Address.State.Abbreviation
Age
Solution 2 - C#
Here is a method that lets you get the string representation, even when you have nested properties:
public static string GetPropertySymbol<T,TResult>(Expression<Func<T,TResult>> expression)
{
return String.Join(".",
GetMembersOnPath(expression.Body as MemberExpression)
.Select(m => m.Member.Name)
.Reverse());
}
private static IEnumerable<MemberExpression> GetMembersOnPath(MemberExpression expression)
{
while(expression != null)
{
yield return expression;
expression = expression.Expression as MemberExpression;
}
}
If you are still on .NET 3.5, you need to stick a ToArray() after the call to Reverse(), because the overload of String.Join that takes an IEnumerable was first added in .NET 4.
Solution 3 - C#
For "FirstName" from p => p.FirstName
Expression<Func<TModel, TProperty>> expression; //your given expression
string fieldName = ((MemberExpression)expression.Body).Member.Name; //watch out for runtime casting errors
I will suggest you check out the ASP.NET MVC 2 code (from aspnet.codeplex.com) as it has similar API for Html helpers... Html.TextBoxFor( p => p.FirstName ) etc
Solution 4 - C#
Another simple approach is to use System.Web.Mvc.ExpressionHelper.GetExpressionText method. In my next blow I will write more in detail. Have a look to http://carrarini.blogspot.com/.
Solution 5 - C#
I wrote a little code for this, and it seemed to work.
Given the following three class definitions:
class Person {
public string FirstName { get; set; }
public string LastName { get; set; }
public Address Address { get; set; }
}
class State {
public string Abbreviation { get; set; }
}
class Address {
public string City { get; set; }
public State State { get; set; }
}
The following method will give you the full property path
static string GetFullSortName<TModel, TProperty>(Expression<Func<TModel, TProperty>> expression) {
var memberNames = new List<string>();
var memberExpression = expression.Body as MemberExpression;
while (null != memberExpression) {
memberNames.Add(memberExpression.Member.Name);
memberExpression = memberExpression.Expression as MemberExpression;
}
memberNames.Reverse();
string fullName = string.Join(".", memberNames.ToArray());
return fullName;
}
For the two calls:
fullName = GetFullSortName<Person, string>(p => p.FirstName);
fullName = GetFullSortName<Person, string>(p => p.Address.State.Abbreviation);
Solution 6 - C#
ExpressionHelper source from MVC is here
https://github.com/ASP-NET-MVC/aspnetwebstack/blob/master/src/System.Web.Mvc/ExpressionHelper.cs
Just take this class - and you'll avoid taking dependency on MVC and get special edge cases handled for you.
Disclaimer: Not sure how licensing works just taking a class like this - but seems pretty innocuous
Solution 7 - C#
Based on this and several related questions/answers here, here's the simple method I'm using:
protected string propertyNameFromExpression<T>(Expression<Func<T, object>> prop)
{
// http://stackoverflow.com/questions/2789504/get-the-property-as-a-string-from-an-expressionfunctmodel-tproperty
// http://stackoverflow.com/questions/767733/converting-a-net-funct-to-a-net-expressionfunct
// http://stackoverflow.com/questions/793571/why-would-you-use-expressionfunct-rather-than-funct
MemberExpression expr;
if (prop.Body is MemberExpression)
// .Net interpreted this code trivially like t => t.Id
expr = (MemberExpression)prop.Body;
else
// .Net wrapped this code in Convert to reduce errors, meaning it's t => Convert(t.Id) - get at the
// t.Id inside
expr = (MemberExpression)((UnaryExpression)prop.Body).Operand;
string name = expr.Member.Name;
return name;
}
You can use it simply like:
string name = propertyNameFromExpression(t => t.Id); // returns "Id"
This method however does less error checking than others posted here - basically it takes for granted it's called properly, which may not be a safe assumption in your application.
Solution 8 - C#
The code that I have working 100% now is as follows, but I don't really understand what it's doing (despite the fact that I modified it to make it handle these deeply-nested scenarios thanks to the debugger).
internal static string MemberWithoutInstance(this LambdaExpression expression)
{
var memberExpression = expression.ToMemberExpression();
if (memberExpression == null)
{
return null;
}
if (memberExpression.Expression.NodeType == ExpressionType.MemberAccess)
{
var innerMemberExpression = (MemberExpression) memberExpression.Expression;
while (innerMemberExpression.Expression.NodeType == ExpressionType.MemberAccess)
{
innerMemberExpression = (MemberExpression) innerMemberExpression.Expression;
}
var parameterExpression = (ParameterExpression) innerMemberExpression.Expression;
// +1 accounts for the ".".
return memberExpression.ToString().Substring(parameterExpression.ToString().Length + 1);
}
return memberExpression.Member.Name;
}
internal static MemberExpression ToMemberExpression(this LambdaExpression expression)
{
var memberExpression = expression.Body as MemberExpression;
if (memberExpression == null)
{
var unaryExpression = expression.Body as UnaryExpression;
if (unaryExpression != null)
{
memberExpression = unaryExpression.Operand as MemberExpression;
}
}
return memberExpression;
}
public static string PropertyToString<TModel, TProperty>(this Expression<Func<TModel, TProperty>> source)
{
return source.MemberWithoutInstance();
}
This solution handles it when my expression is of type Expression<Func<TModel,object>> and I pass all sorts of object types in for my parameters. When I do this, my x => x.Age expression gets turned into x => Convert(x.Age) and that breaks the other solutions on here. I don't understand what in this handles the Convert part, though. :-/
Solution 9 - C#
Cross-posting from https://stackoverflow.com/questions/671968/retrieving-property-name-from-lambda-expression/17220748#17220748
As the question alluded to, the sneaky answer is that if you call expression.ToString(), it will give you something like:
"o => o.ParentProperty.ChildProperty"
which you can then just substring from the first period.
Based on some LinqPad tests, performance was comparable.