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Code Contracts

Code contracts provide a way to specify preconditions, postconditions, and object invariants in your code. Preconditions are requirements that must be met when entering a method or property. Postconditions describe expectations at the time the method or property code exits. Object invariants describe the expected state for a class that is in a good state.

Code contracts include classes for marking your code, a static analyzer for compile-time analysis, and a runtime analyzer. The classes for code contracts can be found in the System.Diagnostics.Contracts namespace.

The benefits of code contracts include the following:

  • Improved testing: Code contracts provide static contract verification, runtime checking, and documentation generation.

  • Automatic testing tools: You can use code contracts to generate more meaningful unit tests by filtering out meaningless test arguments that do not satisfy preconditions.

  • Static verification: The static checker can decide whether there are any contract violations without running the program. It checks for implicit contracts, such as null dereferences and array bounds, and explicit contracts.

  • Reference documentation: The documentation generator augments existing XML documentation files with contract information. There are also style sheets that can be used with Sandcastle so that the generated documentation pages have contract sections.

All .NET Framework languages can immediately take advantage of contracts; you do not have to write a special parser or compiler. A Visual Studio add-in lets you specify the level of code contract analysis to be performed. The analyzers can confirm that the contracts are well-formed (type checking and name resolution) and can produce a compiled form of the contracts in Microsoft intermediate language (MSIL) format. Authoring contracts in Visual Studio lets you take advantage of the standard IntelliSense provided by the tool.

Most methods in the contract class are conditionally compiled; that is, the compiler emits calls to these methods only when you define a special symbol, CONTRACTS FULL, by using the #define directive. CONTRACTS FULL lets you write contracts in your code without using #ifdef directives; you can produce different builds, some with contracts, and some without.

For tools and detailed instructions for using code contracts, see Code Contracts on the MSDN DevLabs Web site.

You can express preconditions by using the Contract.Requires method. Preconditions specify state when a method is invoked. They are generally used to specify valid parameter values. All members that are mentioned in preconditions must be at least as accessible as the method itself; otherwise, the precondition might not be understood by all callers of a method. The condition must have no side-effects. The run-time behavior of failed preconditions is determined by the runtime analyzer.

For example, the following precondition expresses that parameter x must be non-null.

Contract.Requires( x != null );

If your code must throw a particular exception on failure of a precondition, you can use the generic overload of Requires as follows.

Contract.Requires<ArgumentNullException>( x != null, "x" );

Legacy Requires Statements

Most code contains some parameter validation in the form of if-then-throw code. The contract tools recognize these statements as preconditions in the following cases:

When if-then-throw statements appear in this form, the tools recognize them as legacy requires statements. If no other contracts follow the if-then-throw sequence, end the code with the Contract.EndContractBlock method.

if ( x == null ) throw new ...
Contract.EndContractBlock(); // All previous "if" checks are preconditions

Note that the condition in the preceding test is a negated precondition. (The actual precondition would be x != null.) A negated precondition is highly restricted: It must be written as shown in the previous example; that is, it should contain no else clauses, and the body of the then clause must be a single throw statement. The if test is subject to both purity and visibility rules (see Usage Guidelines), but the throw expression is subject only to purity rules. However, the type of the exception thrown must be as visible as the method in which the contract occurs.

Postconditions are contracts for the state of a method when it terminates. The postcondition is checked just before exiting a method. The run-time behavior of failed postconditions is determined by the runtime analyzer.

Unlike preconditions, postconditions may reference members with less visibility. A client may not be able to understand or make use of some of the information expressed by a postcondition using private state, but this does not affect the client's ability to use the method correctly.

Standard Postconditions

You can express standard postconditions by using the Ensures method. Postconditions express a condition that must be true upon normal termination of the method.

Contract.Ensures( this .F > 0 );

Exceptional Postconditions

Exceptional postconditions are postconditions that should be true when a particular exception is thrown by a method. You can specify these postconditions by using the Contract.EnsuresOnThrow method, as the following example shows.

Contract.EnsuresOnThrow<T>( this.F > 0 );

The argument is the condition that must be true whenever an exception that is a subtype of T is thrown.

There are some exception types that are difficult to use in an exceptional postcondition. For example, using the type Exception for T requires the method to guarantee the condition regardless of the type of exception that is thrown, even if it is a stack overflow or other impossible-to-control exception. You should use exceptional postconditions only for specific exceptions that might be thrown when a member is called, for example, when an InvalidTimeZoneException is thrown for a TimeZoneInfo method call.

Special Postconditions

The following methods may be used only within postconditions:

  • You can refer to method return values in postconditions by using the expression Contract. Result<T>(), where T is replaced by the return type of the method. When the compiler is unable to infer the type, you must explicitly provide it. For example, the C# compiler is unable to infer types for methods that do not take any arguments, so it requires the following postcondition: Contract.Ensures(0 < Contract.Result<int>()) Methods with a return type of void cannot refer to Contract. Result<T>() in their postconditions.

  • A prestate value in a postcondition refers to the value of an expression at the start of a method or property. It uses the expression Contract.OldValue<T>(e), where T is the type of e. You can omit the generic type argument whenever the compiler is able to infer its type. (For example, the C# compiler always infers the type because it takes an argument.) There are several restrictions on what can occur in e and the contexts in which an old expression may appear. An old expression cannot contain another old expression. Most importantly, an old expression must refer to a value that existed in the method's precondition state. In other words, it must be an expression that can be evaluated as long as the method's precondition is true. Here are several instances of that rule.

    • The value must exist in the method's precondition state. In order to reference a field on an object, the preconditions must guarantee that that object is always non-null.

    • You cannot refer to the method's return value in an old expression:

      Contract.OldValue(Contract.Result<int>() + x) // ERROR
    • You cannot refer to out parameters in an old expression.

    • An old expression cannot depend on the bound variable of a quantifier if the range of the quantifier depends on the return value of the method:

      Contract. ForAll (0,Contract. Result<int>(),
      i => Contract.OldValue(xs[i]) > 3 ); // ERROR
    • An old expression cannot refer to the parameter of the anonymous delegate in a ForAll or Exists call unless it is used as an indexer or argument to a method call:

      Contract. ForAll (0, xs .Length, i => Contract.OldValue(xs[i]) > 3); // OK
      Contract. ForAll (0, xs .Length, i => Contract.OldValue(i) > 3 ); // ERROR
    • An old expression cannot occur in the body of an anonymous delegate if the value of the old expression depends on any of the parameters of the anonymous delegate, unless the anonymous delegate is an argument to the ForAll or Exists method:

      Method( ... (T t) => Contract.OldValue(... t ...) ... ); // ERROR
    • Out parameters present a problem because contracts appear before the body of the method, and most compilers do not allow references to out parameters in postconditions. To solve this problem, the Contract class provides the ValueAtReturn(Of T) method, which allows a postcondition based on an out parameter.

      public void OutParam(out int x) f
      Contract.Ensures(Contract.ValueAtReturn(out x) == 3);
      x = 3;

      As with the OldValue(Of T) method, you can omit the generic type parameter whenever the compiler is able to infer its type. The contract rewriter replaces the method call with the value of the out parameter. The ValueAtReturn(Of T) method may appear only in postconditions. The argument to the method must be an out parameter or a field of a structure out parameter. The latter is also useful when referring to fields in the postcondition of a structure constructor.


      Currently, the code contract analysis tools do not check whether out parameters are initialized properly and disregard their mention in the postcondition. Therefore, in the previous example, if the line after the contract had used the value of x instead of assigning an integer to it, a compiler would not issue the correct error. However, on a build where the CONTRACTS FULL preprocessor symbol is not defined (such as a release build), the compiler will issue an error.

Object invariants are conditions that should be true for each instance of a class whenever that object is visible to a client. They express the conditions under which the object is considered to be correct.

The invariant methods are identified by being marked with the ContractInvariantMethodAttribute attribute. The invariant methods must contain no code except for a sequence of calls to the Invariant method, each of which specifies an individual invariant, as shown in the following example.

protected void ObjectInvariant () 
Contract.Invariant ( this.y >= 0 );
Contract.Invariant ( this.x > this.y );

Invariants are conditionally defined by the CONTRACTS FULL preprocessor symbol. During run-time checking, invariants are checked at the end of each public method. If an invariant mentions a public method in the same class, the invariant check that would normally happen at the end of that public method is disabled. Instead, the check occurs only at the end of the outermost method call to that class. This also happens if the class is re-entered because of a call to a method on another class. Invariants are not checked for object finalizers or for any methods that implement the Dispose method.

Contract Ordering

The following table shows the order of elements you should use when you write method contracts.

If-then-throw statements

Backward-compatible public preconditions


All public preconditions.


All public (normal) postconditions.


All public exceptional postconditions.


All private/internal (normal) postconditions.


All private/internal exceptional postconditions.


If using if-then-throw style preconditions without any other contracts, place a call to EndContractBlock to indicate that all previous if checks are preconditions.


All methods that are called within a contract must be pure; that is, they must not update any preexisting state. A pure method is allowed to modify objects that have been created after entry into the pure method.

Code contract tools currently assume that the following code elements are pure:

  • Methods that are marked with the PureAttribute.

  • Types that are marked with the PureAttribute (the attribute applies to all the type's methods).

  • Property get accessors.

  • Operators (static methods whose names start with "op", and that have one or two parameters and a non-void return type).

  • Any method whose fully qualified name begins with "System.Diagnostics.Contracts.Contract", "System.String", "System.IO.Path", or "System.Type".

  • Any invoked delegate, provided that the delegate type itself is attributed with the PureAttribute. The delegate types System.Predicate(Of T) and System.Comparison(Of T) are considered pure.


All members mentioned in a contract must be at least as visible as the method in which they appear. For example, a private field cannot be mentioned in a precondition for a public method; clients cannot validate such a contract before they call the method. However, if the field is marked with the ContractPublicPropertyNameAttribute, it is exempt from these rules.

The following example shows the use of code contracts.

Imports System
Imports System.Diagnostics.Contracts

' An IArray is an ordered collection of objects.    
<ContractClass(GetType(IArrayContract))> _
Public Interface IArray
    ' The Item property provides methods to read and edit entries in the array.

    Default Property Item(ByVal index As Integer) As [Object]

    ReadOnly Property Count() As Integer

    ' Adds an item to the list.  
    ' The return value is the position the new element was inserted in.
    Function Add(ByVal value As Object) As Integer

    ' Removes all items from the list.
    Sub Clear()

    ' Inserts value into the array at position index.
    ' index must be non-negative and less than or equal to the 
    ' number of elements in the array.  If index equals the number
    ' of items in the array, then value is appended to the end.
    Sub Insert(ByVal index As Integer, ByVal value As [Object])

    ' Removes the item at position index.
    Sub RemoveAt(ByVal index As Integer)
End Interface 'IArray

<ContractClassFor(GetType(IArray))> _
Friend MustInherit Class IArrayContract
    Implements IArray

    Function Add(ByVal value As Object) As Integer Implements IArray.Add
        ' Returns the index in which an item was inserted.
        Contract.Ensures(Contract.Result(Of Integer)() >= -1) '
        Contract.Ensures(Contract.Result(Of Integer)() < CType(Me, IArray).Count) '
        Return 0

    End Function 'IArray.Add

    Default Property Item(ByVal index As Integer) As Object Implements IArray.Item
            Contract.Requires(index >= 0)
            Contract.Requires(index < CType(Me, IArray).Count)
            Return 0 '
        End Get
        Set(ByVal value As [Object])
            Contract.Requires(index >= 0)
            Contract.Requires(index < CType(Me, IArray).Count)
        End Set
    End Property

    Public ReadOnly Property Count() As Integer Implements IArray.Count
            Contract.Requires(Count >= 0)
            Contract.Requires(Count <= CType(Me, IArray).Count)
            Return 0 '
        End Get
    End Property

    Sub Clear() Implements IArray.Clear
        Contract.Ensures(CType(Me, IArray).Count = 0)

    End Sub 'IArray.Clear

    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

    Sub RemoveAt(ByVal index As Integer) Implements IArray.RemoveAt
        Contract.Requires(index >= 0)
        Contract.Requires(index < CType(Me, IArray).Count)
        Contract.Ensures(CType(Me, IArray).Count = Contract.OldValue(CType(Me, IArray).Count) - 1)

    End Sub 'IArray.RemoveAt
End Class 'IArrayContract