Contract annotations
Contract annotations let you define expected outputs for given inputs, or put in other words, define dependencies between reference type and boolean arguments of a function and its return value. The mechanism of contract annotations allows creating APIs that could be consumed in easier and safer way.
You can implement contract annotations by decorating your functions with the [ContractAnnotationAttribute]
. If you want to do it in your source code, reference the JetBrains.Annotations namespace. You can also annotate functions in existing binary modules using external annotations.
How it works
To quickly understand how and why you could use contract annotations, look at the example below.
In this example, we decorated the function Adjust
with the contract annotation attribute. The attribute argument in this case means, that a null argument always yields a null return. You can easily read the code of this example to see that the function works this way, but in real-life code this dependency might not be that obvious. Anyway, the main thing here is the contract annotation attribute that describes how the function handles the input value.
When we call the Adjust
function with a 'null' argument, ReSharper finds and highlights a bunch of issues at once. First of all, it highlights the function call with a null argument, warning that this expression is always null. Then, it keeps tracking the adjusted
variable that was initialized with this expression, and now is also 'null'. When we check the adjusted
variable for inequality to 'null', ReSharper warns us again that this comparison is always false. Finally, ReSharper greys out code in the 'if' statement as unreachable:
Syntax of contract annotations
Use the following syntax to specify input-output dependencies for contract annotations:
The input
can be:
null
/notnull
for reference type parameterstrue
/false
for boolean parameters.
The output
can be
null
/notnull
/canbenull
for the return value of reference typetrue
/false
for the return value of boolean typehalt
|stop
|void
|nothing
(which are interchangeable) to indicate that the function does not return normally. That is, it throws an exception or halts program execution.
The optional boolean forceFullStates
parameter, which is false by default, allows you force the pessimistic mode for the nullability analysis. That is if the method return value is not defined by the contract condition, ReSharper will assume that it might be null.
Remarks:
You can omit
paramName
if there is only one parameter (see the example above)You can omit both
paramName
andinput
if there are no parameters:[ContractAnnotation("=> halt")] public void TerminationMethod()or if the function has the same output independently on the input:
[ContractAnnotation("=> halt")] public void TerminationMethod(object data, bool flag)You can add several conditions for the same parameter:
[ContractAnnotation("input:null => null; input:notnull=>notnull")] public object Transform(object input, bool flag)You can reverse conditions, that is
input => output
is equal tooutput <= input
:[ContractAnnotation("null <= surname:null")] public string GetName(string surname)You can also specify expected values for 'out' parameters. If you want to specify both a return value and an 'out' parameter for the same input condition, use the comma:
[ContractAnnotation("s:null => false,result:null")] public bool TryParse(string s, out object result)
Validation of contract conditions
If you decorate functions with contract annotation in the source code, ReSharper verifies contract conditions according to the function signature. If the contract annotation does not suit function parameters, ReSharper displays a warning:
The same happens if the contract annotation does not suit the return value.