Behold the following example with System.Nullable<T>
in C#.
int x = 5;
int? y = 3;
int? result = x + y; //no compiler warning
It makes sense that the compiler can figure out that T
is an int
, thus it can use the operator.
Also in
int x = 5;
int? y = 3;
bool result = x == y; //no compiler warning
it makes sense, if x
was null
, the expression would be false
. The compiler doesn't mind.
Now I'm trying to create a lookalike Nullable<T>
class. Let's call it Lookable<T>
.
[Serializable]
public struct Lookable<T> where T : struct
{
public Lookable(T value)
{
Value = value;
}
public T Value { get; }
public override bool Equals(object other)
{
return other != null && Value.Equals(other);
}
public override int GetHashCode()
{
return Value.GetHashCode();
}
public override string ToString()
{
return Value.ToString();
}
public static implicit operator Lookable<T>(T value)
{
return new Lookable<T>(value);
}
public static explicit operator T(Lookable<T> value)
{
return value.Value;
}
}
The idea here is straight from .NET's source code . In my case I'm just omitting the HasValue
property. Now this example would work:
int x = 6;
Lookable<int> y = x;
Lookable<int> z = 4;
The compiler can infer the types here because of the implicit operator
correct?
What I don't understand is that this example will make the compiler unhappy:
int x = 5;
Lookable<int> y = 3;
var result1 = x + y; //compile error
var result2 = x == y; //compile error
The compiler is giving me the message:
Operator cannot be applied to operands of type '
int
' and 'Lookable<int>
'.
Why not? And why is it possible with Nullable<T>
? I can't find it anywhere in the source code. Would it also be possible for Lookable<T>
?
The code for this isn't in Nullable<T>
- it is in the C# compiler, in particular "lifted operators" in the specification, and how they apply specifically to System.Nullable<T>
. The specification references are in this answer .
You cannot reproduce the Nullable<T>
behaviour in your own types. It has special handling by both the compiler and the runtime (boxing, etc).
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