Question

I’m writing a class that holds a variable of any type. To do so, I store both the variable (as an object reference) and its Type. When I try to cast the object back to the correct type, though, I get error CS0118, because I’m using a field (which is of type Type) as a type.

Here is my class:

public class Node
{
    Type m_oType = null;
    public Type Type 
    { 
        get { return m_oType; }
        set { m_oType = value; }
    }

    object m_oValue = null;
    public object Value
    {
        get { return m_oValue; }
        set
        {
            if (m_oValue == null)
            {
                if (value is m_oType) // ERROR CS0118
                {
                    m_oValue = value;
                }
            }
        }
    }
}

I’ve tried to search online for a way to do this (i.e., using cast operators, as and is), but I keep getting the same basic tutorials about casting variables. Can someone give me a pointer as to how I can achieve this? Thanks.

Answer 1

I suspect you want something like this (but read on):

if (m_oType.IsAssignableFrom(value.GetType()))

Note that we’re calling it on m_oType, not passing m_oType to it. From the docs on IsAssignableFrom‘s return value, where c is the parameter:

true if c and the current Type represent the same type, or if the current Type is in the inheritance hierarchy of c, or if the current Type is an interface that c implements, or if c is a generic type parameter and the current Type represents one of the constraints of c.

For example, typeof(object).IsAssignableFrom(typeof(string)) returns true because object is in the inheritance hierarchy of string.

EDIT: As noted, that will break if either m_oType is null or value is null. We can get around value being null easily enough, but it’s not clear what you’d expect it to do if m_oType is null. Perhaps you should prevent that in the setter for the Type property (and the constructor)? Then use either:

// This will always store a null
if (value == null || m_oType.IsAssignableFrom(value.GetType()))

// This will never store a null
if (value != null && m_oType.IsAssignableFrom(value.GetType()))