Question

I have written a generic class

 class MyClass : MyClass<string> { } //this line is in error

 class MyClass<OutputBaseType> : MyClass<OutputBaseType, List<OutputBaseType>>
     where OutputBaseType : new() //this new constraint is needed
 {}
 class MyClass<OutputBaseType,OutputType> 
    where OutputType : ICollection<OutputBaseType>, new()
    where OutputBaseType : new()
 {
     public void Method(OutputBaseType z)
     {
         if (z!=null) { ... }
     }
 ...
 }

Because string has no default constructor the first line will now work. If I remove this line and modify clients to explicitly define string, then it does not work for the same reason.

If I remove where OutputBaseType : new() and the other new() then it works with OutputBaseType of string but not with OutputType of dictionary<...>. This is because KeyValuePair<> used by dictionary is a struct not a class, so can not be null. (I don’t like null usually but need it here)

Is there a way to get this to work?

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Amendments:

I have removed new constraint on OutputBaseType.
Then replaced assigning of null and assigning with default construction, with default(OutputBaseType)

This looks promising but I now have an error in the condition expression on line

OutputBaseType outputItem = ...;
if (outputItem != default(OutputBaseType)) { _Output.Add(outputItem); }

The error is:

Operator '!=' cannot be applied to operands of type 'OutputBaseType' and 'OutputBaseType'   

Answer 1

If I remove where OutputBaseType : new() and the other new() then it works with string OutputBaseType but not with dictionary as OutputType. This is because KeyValuePair<> used by dictionary is a struct not a class, so can not be null. (I don’t like null usually but need it here)

You can simply use default(T) instead of null for value types. The default constructor of value types cannot be user defined, and always initializes all fields to their default value (0, null,…). So you don’t need the new() constraint if you just use default(T) instead of new T(), since for value types they are equivalent, and for reference types you want null which happens to be the default of any reference type.

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== is not available on generic types. You can either use object.Equals (this causes boxing) or EqualityComparer<T>.Default.Equals. I prefer the second one:

EqualityComparer<T>.Default.Equals(outputItem, default(OutputBaseType))