C＃泛型：成员的默认非空值

Question

In this generic "class":

public class Param<ValueType>{
    public ValueType value;
}

When ValueType is string I want value to be non-null for all newly created instances of class without explicitly passing default value through constructor. In my scenario ValueType can only be int , float , bool , string or struct (that doesn't need to be enforced).

How can I do that?

When I try to do this:

public class Param<ValueType> where ValueType: new(){
    public ValueType value = new ValueType();
}

class stops accepting strings.

When I remove new() constraint, I can no longer use new ValueType() (obviously) and value is null for all new instances of Param<string> .

What can I do about it?

The reason why I want this to happen is because I have .Equals() defined which does value.Equals(other.value) , which causes exception to be thrown when value is null.

Answer 1

You could:

public class Param<ValueType>
{
    public ValueType value = typeof(ValueType) == typeof(string) ? (ValueType)(object)string.Empty : default(ValueType);
}

note that default(int) == 0 and so on.

Ah and for the comparison, you should:

bool areEqual = EqualityComparer<ValueType>.Equals(value, other.Value);

It correctly works for null values.

Note even that with generics, this:

if (value == null)
{

}

is perfectly valid code. Clearly it will be true only if ValueType can be null and is null (so for ValueType == int it won't ever happen)