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

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?

Firstly, you've made an 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 Gravell's answer for an explanation of open generic types.

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?