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

Asked 2009-08-28T10:42:24.410
24

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?

c# reflection
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1 Answer

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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.

answered 2009-08-28T11:25:12.187

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