A Go Programming Notebook

Nullable Types Example

The memory for reference types is allocated on the system heap, so it makes sense that reference variables can be null. Value types, in contrast, cannot be assigned a “null” value by default. Still, there are times when you may want to declare a int (for example) and allow it to be set null – one obvious use is in reading and writing data from a database. Enter the Nullable type. Here’s a brief sample that shows how these types work.


using System;

class NullableExample
    static void Main()
    // A basic value type can be assigned to, but cannot be null
    int normal;
    normal = 1;

    // The line below if uncommented produces:
    // error CS0037: Cannot convert null to 'int' because 
    // it is a value type

    // normal = null;

    // Declare a Nullable value
    int? nullable = null;

    // The type? syntax used above is a synonym for Nullable<type>
    // So the following is equivalent
    System.Nullable<system.int32> nullable2 = 5;

    // Display something -- the null for nullable displays nothing
        "normal = {0}, nullable = {1}, nullable2 = {2}", 
        normal, nullable, nullable2);

    // HasValue returns true if the Nullable type has a value
        "nullable.HasValue = {0}, nullable2.HasValue = {1}",
        nullable.HasValue, nullable2.HasValue);

    // Value returns the value or throws an exception if there is no value
        Console.WriteLine("Doesn't display:  {0}", nullable.Value);
    catch (System.InvalidOperationException e)
        Console.WriteLine("Exception caught: " + e);




normal = 1, nullable = , nullable2 = 5
nullable.HasValue = False, nullable2.HasValue = True
Exception caught: System.InvalidOperationException: Nullable object must have a value.
   at System.ThrowHelper.ThrowInvalidOperationException(ExceptionResource resource)
   at NullableExample.Main()