Use of nullable datatype.

// Declaring a nullable variable
int? nullVariable;

// Assign null value to a nullable variable.
nullVariable = null;

// Assign datatype specific value to a nullable variable.
nullVariable = 100;

// Check for null value
if(nullVariable.HasValue)
{
    // Use the nullable variable.
    int actualVale = nullVariable.Value;
    MessageBox.Show("Proper Value available");
}
else
{
    MessageBox.Show("NULL value found");
}