Contract.OldValue<T>(T) Method

Reference

Definition

Namespace:: System.Diagnostics.Contracts

Assemblies:: mscorlib.dll, System.Diagnostics.Contracts.dll

Assemblies:: netstandard.dll, System.Diagnostics.Contracts.dll

Assembly:: System.Diagnostics.Contracts.dll

Assembly:: mscorlib.dll

Assembly:: netstandard.dll

Source:: Contracts.cs

Source:: Contracts.cs

Source:: Contracts.cs

Important

Some information relates to prerelease product that may be substantially modified before it’s released. Microsoft makes no warranties, express or implied, with respect to the information provided here.

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

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.

Applies to

Продукт	Версии
.NET	Core 1.0, Core 1.1, Core 2.0, Core 2.1, Core 2.2, Core 3.0, Core 3.1, 5, 6, 7, 8, 9, 10
.NET Framework	4.0, 4.5, 4.5.1, 4.5.2, 4.6, 4.6.1, 4.6.2, 4.7, 4.7.1, 4.7.2, 4.8, 4.8.1
.NET Standard	2.0, 2.1
UWP	10.0

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