Contract.OldValue<T>(T) Method

Definition

Namespace:

System.Diagnostics.Contracts

Assembly:

System.Diagnostics.Contracts.dll

Source:

Contracts.cs

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

public:
generic <typename T>
 static T OldValue(T value);

public static T OldValue<T> (T value);

static member OldValue : 'T -> 'T

Public Shared Function OldValue(Of T) (value As T) As T

Type Parameters

T

The type of value.

Parameters

value

T

The value to represent (field or parameter).

Returns

T

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

Examples

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

void IArray.Insert(int index, Object value)
{
    Contract.Requires(index >= 0);
    Contract.Requires(index <= ((IArray)this).Count);  // For inserting immediately after the end.
    Contract.Ensures(((IArray)this).Count == Contract.OldValue(((IArray)this).Count) + 1);
}

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

Remarks

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