How to determine whether T is a value type or reference class in generic?

You can use the typeof operator with generic types, so typeof(T) will get the Type reference corresponding to T, and then use the IsValueType property:

if (typeof(T).IsValueType)

Or if you want to include nullable value types as if they were reference types:

// Only true if T is a reference type or nullable value type
if (default(T) == null)