For null values:

Nullable<int> i = null;

Or:

int? i = null;

Or:

var i = (int?)null;

For non-null values:

Nullable<int> i = 0;

Or:

int? i = 0;

int? i = null;

if (i != null)
{
    Console.WriteLine("i is not null");
}
else
{
    Console.WriteLine("i is null");
}

Which is the same as:

if (i.HasValue)
{
    Console.WriteLine("i is not null");
}
else
{
    Console.WriteLine("i is null");
}

Given following nullable int

int? i = 10;

In case default value is needed, you can assign one using null coalescing operator, GetValueOrDefault method or check if nullable int HasValue before assignment.

int j = i ?? 0;
int j = i.GetValueOrDefault(0);
int j = i.HasValue ? i.Value : 0;

The following usage is always unsafe. If i is null at runtime, a System.InvalidOperationException will be thrown. At design time, if a value is not set, you'll get a Use of unassigned local variable 'i' error.

int j = i.Value;

The .GetValueOrDefault() method returns a value even if the .HasValue property is false (unlike the Value property, which throws an exception).

class Program
{
    static void Main()
    {
        int? nullableExample = null;
        int result = nullableExample.GetValueOrDefault();
        Console.WriteLine(result); // will output the default value for int - 0
        int secondResult = nullableExample.GetValueOrDefault(1);
        Console.WriteLine(secondResult) // will output our specified default - 1
        int thirdResult = nullableExample ?? 1;
        Console.WriteLine(secondResult) // same as the GetValueOrDefault but a bit shorter
    }
}

---

Output:

0
1

public bool IsTypeNullable<T>()
{
    return Nullable.GetUnderlyingType( typeof(T) )!=null;
}

public class NullableTypesExample
{
    static int? _testValue;

    public static void Main()
    {
        if(_testValue == null)
            Console.WriteLine("null");
        else
            Console.WriteLine(_testValue.ToString());
    }
}

Output:

null

Any nullable type is a generic type. And any nullable type is a value type.

There are some tricks which allow to effectively use the result of the Nullable.GetUnderlyingType method when creating code related to reflection/code-generation purposes:

public static class TypesHelper {
    public static bool IsNullable(this Type type) {
        Type underlyingType;
        return IsNullable(type, out underlyingType);
    }
    public static bool IsNullable(this Type type, out Type underlyingType) {
        underlyingType = Nullable.GetUnderlyingType(type);
        return underlyingType != null;
    }
    public static Type GetNullable(Type type) {
        Type underlyingType;
        return IsNullable(type, out underlyingType) ? type : NullableTypesCache.Get(type);
    }
    public static bool IsExactOrNullable(this Type type, Func<Type, bool> predicate) {
        Type underlyingType;
        if(IsNullable(type, out underlyingType))
            return IsExactOrNullable(underlyingType, predicate);
        return predicate(type);
    }
    public static bool IsExactOrNullable<T>(this Type type)
        where T : struct {
        return IsExactOrNullable(type, t => Equals(t, typeof(T)));
    }
}

The usage:

Type type = typeof(int).GetNullable();
Console.WriteLine(type.ToString());

if(type.IsNullable())
    Console.WriteLine("Type is nullable.");
Type underlyingType;
if(type.IsNullable(out underlyingType))
    Console.WriteLine("The underlying type is " + underlyingType.Name + ".");
if(type.IsExactOrNullable<int>())
    Console.WriteLine("Type is either exact or nullable Int32.");
if(!type.IsExactOrNullable(t => t.IsEnum))
    Console.WriteLine("Type is neither exact nor nullable enum.");

Output:

System.Nullable`1[System.Int32]
Type is nullable.
The underlying type is Int32.
Type is either exact or nullable Int32.
Type is neither exact nor nullable enum.

PS. The NullableTypesCache is defined as follows:

static class NullableTypesCache {
    readonly static ConcurrentDictionary<Type, Type> cache = new ConcurrentDictionary<Type, Type>();
    static NullableTypesCache() {
        cache.TryAdd(typeof(byte), typeof(Nullable<byte>));
        cache.TryAdd(typeof(short), typeof(Nullable<short>));
        cache.TryAdd(typeof(int), typeof(Nullable<int>));
        cache.TryAdd(typeof(long), typeof(Nullable<long>));
        cache.TryAdd(typeof(float), typeof(Nullable<float>));
        cache.TryAdd(typeof(double), typeof(Nullable<double>));
        cache.TryAdd(typeof(decimal), typeof(Nullable<decimal>));
        cache.TryAdd(typeof(sbyte), typeof(Nullable<sbyte>));
        cache.TryAdd(typeof(ushort), typeof(Nullable<ushort>));
        cache.TryAdd(typeof(uint), typeof(Nullable<uint>));
        cache.TryAdd(typeof(ulong), typeof(Nullable<ulong>));
        //... 
    }
    readonly static Type NullableBase = typeof(Nullable<>);
    internal static Type Get(Type type) {
        // Try to avoid the expensive MakeGenericType method call
        return cache.GetOrAdd(type, t => NullableBase.MakeGenericType(t)); 
    }
}