Method may only be called on a Type for which Type.IsGenericParameter is true.

Question

I am getting this error on a routine which uses reflection to dump some object properties, something like the code below.

MemberInfo[] members = obj.GetType().GetMembers(BindingFlags.Public | BindingFlags.Instance) ;

foreach (MemberInfo m in members)
{
    PropertyInfo p = m as PropertyInfo;
    if (p != null)
    {
       object po = p.GetValue(obj, null);

       ...
    }
}

The actual error is "Exception has been thrown by the target of an invocation" with an inner exception of "Method may only be called on a Type for which Type.IsGenericParameter is true."

At this stage in the debugger obj appears as

  {Name = "SqlConnection" FullName = "System.Data.SqlClient.SqlConnection"}

with the type System.RuntimeType

The method m is {System.Reflection.MethodBase DeclaringMethod}

Note that obj is of type System.RuntimeType and members contains 188 items whereas a simple typeof(System.Data.SqlClient.SqlConnection).GetMembers(System.Reflection.BindingFlags.Public | System.Reflection.BindingFlags.Instance) only returns 65.

I tried checking isGenericParameter on both obj and p.PropertyType, but this seems to be false for most properties including those where p.GetValue works.

So what exactly is a "Type for which Type.IsGenericParameter is true" and more importantly how do I avoid this error without a try/catch?

Answer 1

So what exactly is a "Type for which Type.IsGenericParameter is true"

That means it is a generic type argument in an open generic type - i.e. where we haven't picked a T yet; for example:

// true
bool isGenParam = typeof(List<>).GetGenericArguments()[0].IsGenericParameter;

// false (T is System.Int32)
bool isGenParam = typeof(List<int>).GetGenericArguments()[0].IsGenericParameter;

So; have you got some open generics hanging around? Perhaps if you can give an example of where you got your obj from?

Answer 2

All the clues are in there. The type of the obj is the Type class itself (or rather the strange RuntimeType derivative).

At the point of failure you loop has arrived the Type class property called DeclaringMethod. However the type that this instance of the Type class is describing is System.Data.SqlClient.SqlConnection which is not a Generic Type of a method.

Hence attempting to invoke the get on DeclaringMethod results in the exception.

The key is you are examining the type of the class Type. Its a bit circular but think of this:-

SqlConnection s = new SqlConnection();
Type t = s.GetType()
Type ouch = t.GetType()

What is class ouch describing?

Answer 3

How do I avoid this error without a try/catch?

You almost certainly can't. When you call p.GetValue, you're calling the getter on that property, which could throw any kind of exception. For example, SqlConnection.ServerVersion will throw an exception if the connection is closed, and you have to handle that.

Where are these extra members coming from?

Your obj already contains the RuntimeType object representing SqlConnection, rather than an instance of SqlConnection. obj.GetMembers() would return the 65 members of the SqlConnection class, but by calling GetType() again, you get the 188 members of RuntimeType.

What is IsGenericParameter?

Instead of representing a class, you can have an instance of RuntimeType that represents a generic parameter to a class or method (The T and TOutput in List<T>.ConvertAll<TOutput>. In this case, DeclaringMethod on the object representing TOutput would let you get a MethodInfo object representing the ConvertAll<> method. However, when the RuntimeType represents a class, the idea of a declaring method makes no sense. That's why reading the property causes the exception that you saw.

Answer 4

Firstly, you've made in incorrect assumption, that is, you've assumed that members has returned the members of an instance of System.Data.SqlClient.SqlConnection, which it has not. What has been returned are the members of an instance of System.Type.

From the MSDN documentation for DeclaringType:

Getting the DeclaringMethod property on a type whose IsGenericParameter property is false throws an InvalidOperationException.

So... it's understandable that an InvalidOperationException is being thrown, since naturally, you're not dealing with an open generic type here. See Marc Gravells answer for an explanation of open generic types.