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ReadOnly (Visual Basic)

Specifies that a variable or property can be read but not written.

Rules

  • Declaration Context. You can use ReadOnly only at module level. This means the declaration context for a ReadOnly element must be a class, structure, or module, and cannot be a source file, namespace, or procedure.

  • Combined Modifiers. You cannot specify ReadOnly together with Static in the same declaration.

  • Assigning a Value. Code consuming a ReadOnly property cannot set its value. But code that has access to the underlying storage can assign or change the value at any time.

    You can assign a value to a ReadOnly variable only in its declaration or in the constructor of a class or structure in which it is defined.

When to Use a ReadOnly Variable

There are situations in which you cannot use a Const Statement (Visual Basic) to declare and assign a constant value. For example, the Const statement might not accept the data type you want to assign, or you might not be able to compute the value at compile time with a constant expression. You might not even know the value at compile time. In these cases, you can use a ReadOnly variable to hold a constant value.

Security note Security Note

If the data type of the variable is a reference type, such as an array or a class instance, its members can be changed even if the variable itself is ReadOnly. The following example illustrates this.

ReadOnly characterArray() As Char = {"x"c, "y"c, "z"c}

Sub changeArrayElement()

characterArray(1) = "M"c

End Sub

When initialized, the array pointed to by characterArray() holds "x", "y", and "z". Because the variable characterArray is ReadOnly, you cannot change its value once it is initialized; that is, you cannot assign a new array to it. However, you can change the values of one or more of the array members. Following a call to the procedure changeArrayElement, the array pointed to by characterArray() holds "x", "M", and "z".

Note that this is similar to declaring a procedure parameter to be ByVal (Visual Basic), which prevents the procedure from changing the calling argument itself but allows it to change its members.

The following example defines a ReadOnly property for the date on which an employee was hired. The class stores the property value internally as a Private variable, and only code inside the class can change that value. However, the property is Public, and any code that can access the class can read the property.

Class employee
    ' Only code inside class employee can change the value of hireDateValue. 
    Private hireDateValue As Date 
    ' Any code that can access class employee can read property dateHired. 
    Public ReadOnly Property dateHired() As Date 
        Get 
            Return hireDateValue
        End Get 
    End Property 
End Class

The ReadOnly modifier can be used in these contexts:

Dim Statement

Property Statement

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