Differences Between Passing an Argument By Value and By Reference 

When you pass one or more arguments to a procedure, each argument corresponds to an underlying programming element in the calling code. You can pass either the value of this underlying element, or a reference to it. This is known as the passing mechanism.

Passing by Value

You pass an argument by value by specifying the ByVal keyword for the corresponding parameter in the procedure definition. When you use this passing mechanism, Visual Basic copies the value of the underlying programming element into a local variable in the procedure. The procedure code does not have any access to the underlying element in the calling code.

Passing by Reference

You pass an argument by reference by specifying the ByRef keyword for the corresponding parameter in the procedure definition. When you use this passing mechanism, Visual Basic gives the procedure a direct reference to the underlying programming element in the calling code.

Passing Mechanism and Element Type

The choice of passing mechanism is not the same as the classification of the underlying element type. Passing by value or by reference refers to what Visual Basic supplies to the procedure code. A value type or reference type refers to how a programming element is stored in memory.

However, the passing mechanism and element type are interrelated. The value of a reference type is a pointer to the data elsewhere in memory. This means that when you pass a reference type by value, the procedure code has a pointer to the underlying element's data, even though it cannot access the underlying element itself. For example, if the element is an array variable, the procedure code does not have access to the variable itself, but it can access the array members.

Ability to Modify

When you pass a nonmodifiable element as an argument, the procedure can never modify it in the calling code, whether it is passed ByVal or ByRef.

For a modifiable element, the following table summarizes the interaction between the element type and the passing mechanism.

Element type	Passed ByVal	Passed ByRef
Value type (contains only a value)	The procedure cannot change the variable or any of its members.	The procedure can change the variable and its members.
Reference type (contains a pointer to a class or structure instance)	The procedure cannot change the variable but can change members of the instance to which it points.	The procedure can change the variable and members of the instance to which it points.

See Also

Tasks

How to: Pass Arguments to a Procedure
How to: Change the Value of a Procedure Argument
How to: Protect a Procedure Argument Against Value Changes
How to: Force an Argument to Be Passed by Value

Concepts

Procedures in Visual Basic
Procedure Parameters and Arguments
Argument Passing By Value and By Reference
Differences Between Modifiable and Nonmodifiable Arguments
Argument Passing by Position and by Name
Value Types and Reference Types