C# - 从 HttpSessionState 读取时出现 InvalidCastException

Question

I have a very simple method that looks like this:

    public void DoSmth(int y)
    {
        XXXXX x = (XXXXX)Session["key"];
        x.DoSmthElse(y);
    }

But I get an exception:

System.InvalidCastException: Unable to cast object of type Z.YYYYY to type Z.XXXXX
   at XYZ.DoSmth(Int32 y)

Exception itself makes sense, but why possibly does it occur? What can I do to prevent it? Why does Session["key"] have type there?

Answer 1

Exception itself makes sense, but why possibly does it occur?

• Because Session["key"] type is not XXXXX ( https://docs.microsoft.com/en-us/dotnet/csharp/programming-guide/types/boxing-and-unboxing )

What can I do to prevent it?

• You can check if Session["key"] contains the right type like this

  if (Session["key"] is XXXXX x) { x.DoSmthElse(y); }

Why does Session["key"] have type there?

• each variable have a type