C# .NET Generics – How Can an Object Not Be Compared to Null?

.netc++generics

I have an 'optional' parameter on a method that is a KeyValuePair. I wanted an overload that passes null to the core method for this parameter, but in the core method, when I want to check if the KeyValuePair is null, I get the following error:

Operator '!=' cannot be applied to operands of type System.Collections.Generic.KeyValuePair<string,object>' and '<null>.

How can I not be allowed to check if an object is null?

Best Answer

KeyValuePair<K,V> is a struct, not a class. It's like doing:

int i = 10;
if (i != null) ...

(Although that is actually legal, with a warning, due to odd nullable conversion rules. The important bit is that the if condition will never be true.)

To make it "optional", you can use the nullable form:

static void Foo(KeyValuePair<object,string>? pair)
{
    if (pair != null)
    {
    }
    // Other code
}

Note the ? in KeyValuePair<object,string>?

Related Solutions

C# Generics – Null or Default Comparison of Generic Argument

To avoid boxing, the best way to compare generics for equality is with EqualityComparer<T>.Default. This respects IEquatable<T> (without boxing) as well as object.Equals, and handles all the Nullable<T> "lifted" nuances. Hence:

if(EqualityComparer<T>.Default.Equals(obj, default(T))) {
    return obj;
}

This will match:

null for classes
null (empty) for Nullable<T>
zero/false/etc for other structs

C# – Object is Not Null but (myObject != null) Returns False

Is the == and/or != operator overloaded for the region object's class?

Now that you've posted the code for the overloads:

The overloads should probably look like the following (code taken from postings made by Jon Skeet and Philip Rieck):

public static bool operator ==(Region r1, Region r2)
{
    if (object.ReferenceEquals( r1, r2)) {
        // handles if both are null as well as object identity
        return true;
    }

    if ((object)r1 == null || (object)r2 == null)
    {
       return false;
    }        

    return (r1.Cmr.CompareTo(r2.Cmr) == 0 && r1.Id == r2.Id);
}

public static bool operator !=(Region r1, Region r2)
{
    return !(r1 == r2);
}

Best Answer

Related Solutions

C# Generics – Null or Default Comparison of Generic Argument

C# – Object is Not Null but (myObject != null) Returns False

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