Why does IsAssignableFrom() not work for int and double?

Question

This is false: typeof(double).IsAssignableFrom(typeof(int))

This is false: typeof(int).IsAssignableFrom(typeof(double))

But this works:

double a = 1.0;
int b = 1;

a = b;

Clearly a double is assignable from an int but the framework IsAssignableFrom() gets it wrong.

Why? Or is this a bug in .NET caused by the special nature of int and double which have no inheritance relationship but are assignable (in one direction)?

Answer 1

C# is providing the implicit conversion from int to double. That's a language decision, not something which .NET will do for you... so from the .NET point of view, double isn't assignable from int.

(As an example of why this is language-specific, F# doesn't perform implicit conversions for you like this - you'd need to explicitly specify the conversion.)

It's worth looking at the documentation for Type.IsAssignableFrom (edited very slightly for readability):

Returns 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. Returns false if none of these conditions are true, or if c is null.

Now apply that to double and int and you'll see it should return false.