Boxing Nullable Value Types in C# // Manski's Dev Log

A nullable value type value (e.g. int?) loses its nullability type information when it’s boxed:

Concrete value:


int? valueType = 42;
object? boxed = valueType;
Console.WriteLine(boxed?.GetType()); // System.Int32

Null value:


int? valueType = null;
object? boxed = valueType;
Console.WriteLine(boxed is null); // true

This also means that using Nullable.GetUnderlyingType() is useless for boxed value types:


public void MyMethod(object? value)
{
    if (value is null)
    {
        return;
    }

    // Always false!
    bool isNullable = Nullable.GetUnderlyingType(value.GetType()) != null;
}