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public static void DoSomething(int Input) { Contract.Requires(Input != null); Contract.Requires(Input != 5); } Before you run the application, go to the Properties page of your project and select the new Code Contracts tab (see Figure 4-4).
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Figure 4-4. New code contract tab Check the box marked Perform Runtime Contract Checking and run the code. You should receive an error message similar to that shown in Figure 4-5.
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Figure 4-5. Alert box showing failure of code contract assumption
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Enabling Code Contract Static Verification (Premium/Ultimate Edition Only)
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To enable static verification, you need to go into the project Properties screen, select the Code Contract tab, and ensure that the static checking option is selected. When you compile your applications now, you should see the contracts are checked at compile time. You might ask why static verification is not always on One reason is that if you are writing unit tests, you might want to pass null values into your methods and ensure that your code handles them correctly. If static verification was always on, you could not run your unit tests.
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You should note that contracts are inherited between classes, but it is not possible to add additional preconditions.
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Architecture
Behind the scenes, code contracts rewrite the generated IL. At a high level, you can divide code contract architecture into three main components: Static method (expresses assumptions and constraints; you will look at them shortly) Binary rewriter (performs runtime checks) Static checker (verifies assumptions at compile time; TFS edition only)
Let s now look at some of the different ways to declare assumptions in the code using the static methods available in code contracts.
Conditions
Code contracts allows you to create three types of conditions: Preconditions Postconditions Invariants
The following sections discuss some of the conditions you might want to utilize (this is by no means an exhaustive list and is being added to in each release).
Preconditions
Preconditions must be true at the start of a method.
Contract.Assert
In debug build, ensures that a condition is true. Contract.Assert(Input != null);
Contract.Assume
Used for static verification and tells code analysis tools to assume that a condition is true (for example, if you are calling a method you have written and you are sure it will never return a null result): Contract.Assume(Input!=null)
Contract.Requires
Ensures that a condition is true before subsequent code is run. The following will ensure that the input parameter is not null: Contract.Requires(input != null);
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Contract.Requires has an overload that allows you to specify and exception type and message to be thrown: Contract.Requires<ArgumentNullException>(Input != null, "input");
Contract.EndContractBlock
The Contract.EndContractBlock statement tells the compiler to treat code as a precondition and allows you to utilize legacy code without converting it to code contracts format: if (Input==null) throw new System.NullReferenceException("input is null"); Contract.EndContractBlock();
NOTE You cannot use EndContractBlock in conjunction with any other preconditions.
Post Conditions
Post conditions are conditions that are true at the exit of your method calls.
Contract.Ensures
Ensures that a condition is true on exit of method: Contract.Ensures(Output != 7);
Contract.EnsuresOnThrow
Ensures that a specific exception type is thrown for a condition: Contract.EnsuresOnThrow<System.IO.IOException>(Input != null);
Contract.ForAll
Allows the iteration through a set to ensure that all members meet a specific condition: Contract.ForAll(MySet, i=> i!=null);
Object Invariants
Object invariants allow you to specify conditions that must always be true for an object and are created by decorating a procedure with the [ContractInvariantMethod] attribute. The following code ensures that the ImportantData variable can never be null:
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[ContractInvariantMethod] void MyInvariant() { Contract.Invariant(ImportantData !=null); }
Code Contract Values
Code contracts offer some useful pseudo variables that can be useful when writing your conditions.
Contract.Result
Contract.Result accesses a value in a condition that will be returned from a function without referring to it directly: Contract.Ensures(Contract.Result<Int32 >() >= -1);
Contract.OldValue
Contract.OldValue represents the values state at the start of the method call. OldValue performs a shallow copy of the specified variable and can be used to see whether a value has changed: Contract.Ensures(Input != Contract.OldValue(Input));
Pure
Methods that are called within a contract should be decorated with the attribute [Pure], which indicates that the method has no side effects (doesn t alter the state of any other objects). If you don t add this attribute, you will receive a warning similar to this: Detected call to impure method 'ConsoleApplication6.Program.Multiply(System.Int32,System.Int32)' in a pure region in method To mark a method as pure, simply add the [Pure] attribute as follows: [Pure] public static double Multiply(int x, int y) { return x * y; }
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