当 c# 的值为 null 时，c# 可以知道 object 的类型吗？

Question

While making some validation, I can across this situation

public override bool IsValid(object value)
{
    if(!(value is IFileUploadInfo) && !(value is IEnumerable<IFileUploadInfo>))
        throw new ArgumentException($"Cannot use {nameof(FileValidationAttribute)} in a property that isn't of type {nameof(IFileUploadInfo)} or {nameof(IEnumerable<IFileUploadInfo>)}");

    // rest ...
}

Where I need to test if a object is of the type that I want, but if value is null , will c# know the type of that object even when it's null ?

Answer 1

You can't. Passing with object in erases what the type was at compile time. Hence you have to do some pattern matching or casting to get it back out. Hence, null doesn't carry any type information with it.

You could make the method generic and then you could get what the type was.

public override bool IsValid<T>(T value)
{
    Type t = typeof(T);
    ... 
    // rest of code
}

Answer 2

No, you can't obtain type information from null with type check.

MyClass mc = null;
object obj = mc;
Console.WriteLine(obj is MyClass); // prints "False"

Docs are clear about it:

The is expression is true if expr isn't null, and...