Lambda and Expression.Call for an extension method

问题:

I need to implement an expression for a method like here:

var prop = Expression.Property(someItem, "Name"); 
var value = Expression.Constant(someConstant);

var contains = typeof(string).GetMethod("Contains", new[] {typeof(string)});
var expression = Expression.Call(prop, contains, value);

But for my extension method:

public static class StringEx
{
    public static bool Like(this string a, string b)
    {
        return a.ToLower().Contains(b.ToLower());
    }
}

Unfortunately, next code throws an ArgumentNullException for a parameter "method":

var like = typeof(string).GetMethod("Like", new[] {typeof(string)});
comparer = Expression.Call(prop, like, value);

What I'm doing wrong?

回答1:

Try this

public class Person
{
    public string Name { get; set; }
}
public static class StringEx
{
    public static bool Like(this string a, string b)
    {
        return a.ToLower().Contains(b.ToLower());
    }
}

Person p = new Person(){Name = "Me"};
var prop = Expression.Property(Expression.Constant(p), "Name");
var value = Expression.Constant("me");
var like = typeof(StringEx).GetMethod("Like", BindingFlags.Static
                        | BindingFlags.Public | BindingFlags.NonPublic);
var comparer = Expression.Call(null, like, prop, value );

var vvv = (Func<bool>) Expression.Lambda(comparer).Compile();
bool isEquals = vvv.Invoke();

回答2:

You can do like this:

var like = typeof(StringEx).GetMethod("Like", new[] {typeof(string), typeof(string)});

comparer = Expression.Call(null, like, prop, value);

You can pass prop as first parameter and value as second parameter like above.

Maybe you will need to get a complete query before apply an extension method.

回答3:

I am not sure, but you can only get an extension method from the static class using reflection. Extension methods are not truly added to the class, therefore can't be retrieved with GetMethod.

回答4:

Use

var like = typeof(StringEx).GetMethod("Like", new[] {typeof(string),typeof(string)});

ie. retrieve it from the extending type, not from the extended type.

回答5:

If you want to get your extension method worked you must do like this:

string str = "some string";
str.Like("second string");