An Object is an instance of a Class , it is stored some where in memory. A reference is what is used to describe the pointer to the memory location where the Object resides. The message "object reference not set to an instance of an object" means that you are referring to an object the does not exist or was deleted or cleaned up. It's usually better to avoid a NullReferenceException than to handle it after it occurs. To prevent the error, objects that could be null should be tested before being used.
if (mClass != null)
{
// Go ahead and use mClass
mClass.property = ...
}
else
{
// Attempting to use mClass here will result in NullReferenceException
}
A NullReferenceException typically reflects developer error and is thrown in the following scenarios:
- Forgotten to instantiate a reference type.
- Forgotten to dimension an array before initializing it.
- Is thrown by a method that is passed null.
- Get a null return value from a method, then call a method on the returned type.
- Using an expression to retrieve a value and, although checking whether the value is null.
- Enumerating the elements of an array that contains reference types, and attempt to process one of the elements.
