OldValue(T) Method

Contract.OldValue(Of T) Method

[ This article is for Windows Phone 8 developers. If you’re developing for Windows 10, see the latest documentation. ]

Represents values as they were at the start of a method or property.

Namespace:  System.Diagnostics.Contracts
Assembly:  mscorlib (in mscorlib.dll)

'Declaration
Public Shared Function OldValue(Of T) ( _
	value As T _
) As T

Type Parameters

T

The type of value.

Parameters

value
Type: T
The value to represent (field or parameter).

Return Value

Type: T
The value of the parameter or field at the start of a method or property.

This method can be used only in the conditional expression for the Ensures contract.

The following example shows the use of the OldValue(Of T) method to ensure that a count has been updated. This code example is part of a larger example provided for the ContractClassAttribute class.


Sub Insert(ByVal index As Integer, ByVal value As [Object]) Implements IArray.Insert
    Contract.Requires(index >= 0)
    Contract.Requires(index <= CType(Me, IArray).Count) ' For inserting immediately after the end.
    Contract.Ensures(CType(Me, IArray).Count = Contract.OldValue(CType(Me, IArray).Count) + 1)

End Sub 'IArray.Insert


Windows Phone OS

Supported in: 8.1, 8.0

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