Question

I’m trying to retrieve MethodInfo for Where method of Enumerable type:

typeof (Enumerable).GetMethod("Where", new Type[] { 
     typeof(IEnumerable<>), 
     typeof(Func<,>) 
})

but get null. What am I doing wrong?

Answer 1

That previous answer works for some cases, however:

• It doesn’t handle nested generic types, such as a parameter type of Action<IEnumerable<T>>. It will treat all Action<> as matches, for example, string.Concat(IEnumerable<string>) and string.Concat<T>(IEnumerable<T>) will both match if searching for "Concat" with type IEnumerable<> on the string type. What is really desirable is handling nested generic types recursively, while treating all generic parameters as matching each other regardless of name while NOT matching concrete types.
• It returns the first method matched rather than throwing an exception if the result is ambiguous, like type.GetMethod() does. So, you might get the method you wanted if you’re lucky, or you might not.
• Sometimes it will be necessary to specify BindingFlags in order to avoid ambiguity, such as when a derived class method ‘hides’ a base class method. You normally want to find base class methods, but not in a specialized case where you know the method you’re looking for is in the derived class. Or, you might know you’re looking for a static vs instance method, public vs private, etc. and don’t want to match if it’s not exact.
• It doesn’t address another major fault with type.GetMethods(), in that it also doesn’t search base interfaces for methods when looking for a method on an interface type. OK, maybe that’s being picky, but it’s another major flaw in GetMethods() that has been a problem for me.
• Calling type.GetMethods() is inefficient, type.GetMember(name, MemberTypes.Method, ...) will return only methods with a matching name instead of ALL methods in the type.
• As a final nit-pick, the name GetGenericMethod() could be misleading, since you might be trying to find a non-generic method that happens to have a type parameter somewhere in a parameter type due to a generic declaring type.

Here’s a version that addresses all of those things, and can be used as a general-purpose replacement for the flawed GetMethod(). Note that two extension methods are provided, one with BindingFlags and one without (for convenience).

/// <summary>
/// Search for a method by name and parameter types.  
/// Unlike GetMethod(), does 'loose' matching on generic
/// parameter types, and searches base interfaces.
/// </summary>
/// <exception cref="AmbiguousMatchException"/>
public static MethodInfo GetMethodExt(  this Type thisType, 
                                        string name, 
                                        params Type[] parameterTypes)
{
    return GetMethodExt(thisType, 
                        name, 
                        BindingFlags.Instance 
                        | BindingFlags.Static 
                        | BindingFlags.Public 
                        | BindingFlags.NonPublic
                        | BindingFlags.FlattenHierarchy, 
                        parameterTypes);
}

/// <summary>
/// Search for a method by name, parameter types, and binding flags.  
/// Unlike GetMethod(), does 'loose' matching on generic
/// parameter types, and searches base interfaces.
/// </summary>
/// <exception cref="AmbiguousMatchException"/>
public static MethodInfo GetMethodExt(  this Type thisType, 
                                        string name, 
                                        BindingFlags bindingFlags, 
                                        params Type[] parameterTypes)
{
    MethodInfo matchingMethod = null;

    // Check all methods with the specified name, including in base classes
    GetMethodExt(ref matchingMethod, thisType, name, bindingFlags, parameterTypes);

    // If we're searching an interface, we have to manually search base interfaces
    if (matchingMethod == null && thisType.IsInterface)
    {
        foreach (Type interfaceType in thisType.GetInterfaces())
            GetMethodExt(ref matchingMethod, 
                         interfaceType, 
                         name, 
                         bindingFlags, 
                         parameterTypes);
    }

    return matchingMethod;
}

private static void GetMethodExt(   ref MethodInfo matchingMethod, 
                                    Type type, 
                                    string name, 
                                    BindingFlags bindingFlags, 
                                    params Type[] parameterTypes)
{
    // Check all methods with the specified name, including in base classes
    foreach (MethodInfo methodInfo in type.GetMember(name, 
                                                     MemberTypes.Method, 
                                                     bindingFlags))
    {
        // Check that the parameter counts and types match, 
        // with 'loose' matching on generic parameters
        ParameterInfo[] parameterInfos = methodInfo.GetParameters();
        if (parameterInfos.Length == parameterTypes.Length)
        {
            int i = 0;
            for (; i < parameterInfos.Length; ++i)
            {
                if (!parameterInfos[i].ParameterType
                                      .IsSimilarType(parameterTypes[i]))
                    break;
            }
            if (i == parameterInfos.Length)
            {
                if (matchingMethod == null)
                    matchingMethod = methodInfo;
                else
                    throw new AmbiguousMatchException(
                           "More than one matching method found!");
            }
        }
    }
}

/// <summary>
/// Special type used to match any generic parameter type in GetMethodExt().
/// </summary>
public class T
{ }

/// <summary>
/// Determines if the two types are either identical, or are both generic 
/// parameters or generic types with generic parameters in the same
///  locations (generic parameters match any other generic paramter,
/// but NOT concrete types).
/// </summary>
private static bool IsSimilarType(this Type thisType, Type type)
{
    // Ignore any 'ref' types
    if (thisType.IsByRef)
        thisType = thisType.GetElementType();
    if (type.IsByRef)
        type = type.GetElementType();

    // Handle array types
    if (thisType.IsArray && type.IsArray)
        return thisType.GetElementType().IsSimilarType(type.GetElementType());

    // If the types are identical, or they're both generic parameters 
    // or the special 'T' type, treat as a match
    if (thisType == type || ((thisType.IsGenericParameter || thisType == typeof(T)) 
                         && (type.IsGenericParameter || type == typeof(T))))
        return true;

    // Handle any generic arguments
    if (thisType.IsGenericType && type.IsGenericType)
    {
        Type[] thisArguments = thisType.GetGenericArguments();
        Type[] arguments = type.GetGenericArguments();
        if (thisArguments.Length == arguments.Length)
        {
            for (int i = 0; i < thisArguments.Length; ++i)
            {
                if (!thisArguments[i].IsSimilarType(arguments[i]))
                    return false;
            }
            return true;
        }
    }

    return false;
}

Note that the IsSimilarType(Type) extension method can be made public and might be useful on its own. I know, the name isn’t great – you’re welcome to come up with a better one, but it might get really long to explain what it does. Also, I added yet another improvement by checking for ‘ref’ and array types (refs are ignored for matching, but arrays dimensions must match).

So, that’s how Microsoft should have done it. It’s really not that hard.

Yeah, I know, you can shorten some of that logic using Linq, but I’m not a huge fan of Linq in low-level routines like this, and also not unless the Linq is about as easy to follow as the original code, which is often NOT the case, IMO.

If you love Linq, and you must, you can replace the inner-most part of IsSimilarType() with this (turns 8 lines into 1):

if (thisArguments.Length == arguments.Length)
    return !thisArguments.Where((t, i) => !t.IsSimilarType(arguments[i])).Any();

One last thing: If you’re looking for a generic method with a generic parameter, such as Method<T>(T, T[]), you’ll have to find a Type which is a generic parameter (IsGenericParameter == true) to pass in for the parameter type (any one will do, because of the ‘wildcard’ matching). However, you can’t just do new Type() – you have to find a real one (or build one with TypeBuilder). To make this easier, I added the public class T declaration, and added logic to IsSimilarType() to check for it and match any generic parameter. If you need a T[], just use T.MakeArrayType(1).