C# generic type constraint for everything nullable

Question

If you are willing to make a runtime check in Foo's constructor rather than having a compile-time check, you can check if the type is not a reference or nullable type, and throw an exception if that's the case.

I realise that only having a runtime check may be unacceptable, but just in case:

public class Foo<T>
{
    private T item;

    public Foo()
    {
        var type = typeof(T);

        if (Nullable.GetUnderlyingType(type) != null)
            return;

        if (type.IsClass)
            return;

        throw new InvalidOperationException("Type is not nullable or reference type.");
    }

    public bool IsNull()
    {
        return item == null;
    }
}

Then the following code compiles, but the last one (foo3) throws an exception in the constructor:

var foo1 = new Foo<int?>();
Console.WriteLine(foo1.IsNull());

var foo2 = new Foo<string>();
Console.WriteLine(foo2.IsNull());

var foo3= new Foo<int>();  // THROWS
Console.WriteLine(foo3.IsNull());

I don't know how to implement equivalent to OR in generics. However I can propose to use default key word in order to create null for nullable types and 0 value for structures:

public class Foo<T>
{
    private T item;

    public bool IsNullOrDefault()
    {
        return Equals(item, default(T));
    }
}

You could also implement you version of Nullable:

class MyNullable<T> where T : struct
{
    public T Value { get; set; }

    public static implicit operator T(MyNullable<T> value)
    {
        return value != null ? value.Value : default(T);
    }

    public static implicit operator MyNullable<T>(T value)
    {
        return new MyNullable<T> { Value = value };
    }
}

class Foo<T> where T : class
{
    public T Item { get; set; }

    public bool IsNull()
    {
        return Item == null;
    }
}

Example:

class Program
{
    static void Main(string[] args)
    {
        Console.WriteLine(new Foo<MyNullable<int>>().IsNull()); // true
        Console.WriteLine(new Foo<MyNullable<int>> {Item = 3}.IsNull()); // false
        Console.WriteLine(new Foo<object>().IsNull()); // true
        Console.WriteLine(new Foo<object> {Item = new object()}.IsNull()); // false

        var foo5 = new Foo<MyNullable<int>>();
        int integer = foo5.Item;
        Console.WriteLine(integer); // 0

        var foo6 = new Foo<MyNullable<double>>();
        double real = foo6.Item;
        Console.WriteLine(real); // 0

        var foo7 = new Foo<MyNullable<double>>();
        foo7.Item = null;
        Console.WriteLine(foo7.Item); // 0
        Console.WriteLine(foo7.IsNull()); // true
        foo7.Item = 3.5;
        Console.WriteLine(foo7.Item); // 3.5
        Console.WriteLine(foo7.IsNull()); // false

        // var foo5 = new Foo<int>(); // Not compile
    }
}

I ran into this issue for a simpler case of wanting a generic static method that could take anything "nullable" (either reference types or Nullables), which brought me to this question with no satisfactory solution. So I came up with my own solution which was relatively easier to solve than the OP's stated question by simply having two overloaded methods, one that takes a T and has the constraint where T : class and another that takes a T? and has where T : struct .

I then realized, that solution can also be applied to this problem to create a solution that is checkable at compile time by making the constructor private (or protected) and using a static factory method:

    //this class is to avoid having to supply generic type arguments 
    //to the static factory call (see CA1000)
    public static class Foo
    {
        public static Foo<TFoo> Create<TFoo>(TFoo value)
            where TFoo : class
        {
            return Foo<TFoo>.Create(value);
        }

        public static Foo<TFoo?> Create<TFoo>(TFoo? value)
            where TFoo : struct
        {
            return Foo<TFoo?>.Create(value);
        }
    }

    public class Foo<T>
    {
        private T item;

        private Foo(T value)
        {
            item = value;
        }

        public bool IsNull()
        {
            return item == null;
        }

        internal static Foo<TFoo> Create<TFoo>(TFoo value)
            where TFoo : class
        {
            return new Foo<TFoo>(value);
        }

        internal static Foo<TFoo?> Create<TFoo>(TFoo? value)
            where TFoo : struct
        {
            return new Foo<TFoo?>(value);
        }
    }

Now we can use it like this:

        var foo1 = new Foo<int>(1); //does not compile
        var foo2 = Foo.Create(2); //does not compile
        var foo3 = Foo.Create(""); //compiles
        var foo4 = Foo.Create(new object()); //compiles
        var foo5 = Foo.Create((int?)5); //compiles

If you want a parameterless constructor, you won't get the nicety of overloading, but you can still do something like this:

    public static class Foo
    {
        public static Foo<TFoo> Create<TFoo>()
            where TFoo : class
        {
            return Foo<TFoo>.Create<TFoo>();
        }

        public static Foo<TFoo?> CreateNullable<TFoo>()
            where TFoo : struct
        {
            return Foo<TFoo?>.CreateNullable<TFoo>();
        }
    }

    public class Foo<T>
    {
        private T item;

        private Foo()
        {
        }

        public bool IsNull()
        {
            return item == null;
        }

        internal static Foo<TFoo> Create<TFoo>()
            where TFoo : class
        {
            return new Foo<TFoo>();
        }

        internal static Foo<TFoo?> CreateNullable<TFoo>()
            where TFoo : struct
        {
            return new Foo<TFoo?>();
        }
    }

And use it like this:

        var foo1 = new Foo<int>(); //does not compile
        var foo2 = Foo.Create<int>(); //does not compile
        var foo3 = Foo.Create<string>(); //compiles
        var foo4 = Foo.Create<object>(); //compiles
        var foo5 = Foo.CreateNullable<int>(); //compiles

There are few disadvantages to this solution, one is that you may prefer using 'new' to construct objects. Another is that you won't be able to use Foo<T> as a generic type argument for a type constraint of something like: where TFoo: new(). Finally is the bit of extra code you need here which would increase especially if you need multiple overloaded constructors.

As mentioned, you cannot have a compile-time check for it. Generic constraints in .NET are severely lacking, and do not support most scenarios.

However I consider this to be a better solution for run-time checking. It can be optimized at JIT compilation time, since they're both constants.

public class SomeClass<T>
{
    public SomeClass()
    {
        // JIT-compile time check, so it doesn't even have to evaluate.
        if (default(T) != null)
            throw new InvalidOperationException("SomeClass<T> requires T to be a nullable type.");

        T variable;
        // This still won't compile
        // variable = null;
        // but because you know it's a nullable type, this works just fine
        variable = default(T);
    }
}

Such a type constraint is not possible. According to the documentation of type constraints there is not constraint that captures both the nullable and the reference types. Since constraints can only be combined in a conjunction, there is no way to create such a constraint by combination.

You can, however, for your needs fall back to an unconstraint type parameter, since you can always check for == null. If the type is a value type the check will just always evaluate to false. Then you'll possibly get the R# warning "Possible compare of value type with null", which is not critical, as long as the semantics is right for you.

An alternative could be to use

object.Equals(value, default(T))

instead of the null check, since default(T) where T : class is always null. This, however, means that you cannot distinguish weather a non-nullable value has never been set explicitly or was just set to its default value.