Get the name of a method using an expression

x => x.DoSomething

In order to make this compilable I see only two ways:

1. Go non-generic way and specify it's parameter as Action<string, string>
2. Specify Action<string, string> as your target delegate type by yourself: GetMethodInfo<IMyInteface>(x => new Action<string,string>(x.DoSomething))

if you are ok to go with second one, which allows you to omit arguments then you can write your GetMethodInfo method as follows:

    MemberInfo GetMethodInfo<T>(Expression<Func<T, Delegate>> expression)
    {
        var unaryExpression = (UnaryExpression) expression.Body;
        var methodCallExpression = (MethodCallExpression) unaryExpression.Operand;
        var methodInfoExpression = (ConstantExpression) methodCallExpression.Arguments.Last();
        var methodInfo = (MemberInfo) methodInfoExpression.Value;
        return methodInfo;
    }

It works for your interface, but probably some generalization will be required to make this working with any method, that's up to you.

The following is compatible with .NET 4.5:

public static string MethodName(LambdaExpression expression)
{
    var unaryExpression = (UnaryExpression)expression.Body;
    var methodCallExpression = (MethodCallExpression)unaryExpression.Operand;
    var methodCallObject = (ConstantExpression)methodCallExpression.Object;
    var methodInfo = (MethodInfo)methodCallObject.Value;

    return methodInfo.Name;
}

You can use it with expressions like x => x.DoSomething, however it would require some wrapping into generic methods for different types of methods.

Here is a backwards-compatible version:

private static bool IsNET45 = Type.GetType("System.Reflection.ReflectionContext", false) != null;

public static string MethodName(LambdaExpression expression)
{
    var unaryExpression = (UnaryExpression)expression.Body;
    var methodCallExpression = (MethodCallExpression)unaryExpression.Operand;
    if (IsNET45)
    {
        var methodCallObject = (ConstantExpression)methodCallExpression.Object;
        var methodInfo = (MethodInfo)methodCallObject.Value;
        return methodInfo.Name;
    }
    else
    {
        var methodInfoExpression = (ConstantExpression)methodCallExpression.Arguments.Last();
        var methodInfo = (MemberInfo)methodInfoExpression.Value;
        return methodInfo.Name;
    }
}

Check this sample code on Ideone. Note, that Ideone does not have .NET 4.5.