ASP.NET Core WebAPI Action Parameters Validation

There are two types of guards:

The enforcers, as the name suggests, enforce an argument to be within the check limits by throwing an EnforcerException if they fail. By adding the EnforcerMiddleware each exception is handled and a 400 BadRequest is returned. You can also create your own Exception by implementing the IEnforcerException interface, or inheriting from the EnforcerException type.
The Checks, a simpler version that just check and return true or false depending on the check success or failure

It is generally a very bad idea to use exceptions to control flow, and the use of the enforcers can contribute to this anti-pattern. Use them specifically for what they are designed, preferably at the top of each action, clearly separated from the rest of your code! Read this for more details: https://web.archive.org/web/20140430044213/http://c2.com/cgi-bin/wiki?DontUseExceptionsForFlowControl

Usage

Guards sample

public async Task<IActionResult> Get(Guid projectId, int skip, int take, string query)
{
	//Enforcers
    	Guard.Object.IsNotNullOrDefault(projectId, "ProjectId");
    	Guard.Numeric.IsGreaterOrEqualThan(skip, 0, "Skip");	
	
	//Checks
	if( !Check.Numeric.IsBetweenInclusive(0, 1, 3, "RandomNumber") )
	     //Check failed
	//...
}

Enforcers included

Enforcers throw an IEnforcerException when something is wrong. Please use the Enforcer middleware to transparently catch this exception and return a 400

Bool Enforcers

IsTrue(bool value, string message)
IsFalse(bool value, string message)

Numeric Enforcers

IsGreaterThan<T>(T value, T than, string message) where T : IComparable
IsGreaterOrEqualThan<T>(T value, T than, string message) where T : IComparable
IsSmallerThan<T>(T value, T than, string message) where T : IComparable
IsSmallerOrEqualThan<T>(T value, T than, string message) where T : IComparable
IsBetweenInclusive<T>(T value, T from, T to, string message) where T : IComparable
IsBetweenExclusive<T>(T value, T from, T to, string message) where T : IComparable

Object Enforcers

AreEqual<T>(T obj1, T obj2, string message) where T : IComparable
AreNotEqual<T>(T obj1, T obj2, string message) where T : IComparable
IsNullOrDefault<T>(T value, string message)
IsNotNullOrDefault<T>(T value, string message)
IsDefined<T>(T value, string message)
IsNotDefined<T>(T value, string message)

String Enforcers

IsNullOrEmpty(string value, string message)
IsNotNullOrEmpty(string value, string message)
IsNullOrWhiteSpace(string value, string message)
IsNotNullOrWhiteSpace(string value, string message)
IsNull(string variable, string message)
IsNotNull(string variable, string message)

Enforcer middleware

In order for the enforcers to work correctly, you need to add the enforcer middleware in the pipeline. The middleware will handle all exceptions that implement the IEnforcerException interface.

Add the handling middleware in Startup.cs

public void Configure(IApplicationBuilder app, IHostingEnvironment env) {
    //...
    app.AddEnforcerMiddleware();//Position is important, it has to be before other middlewares
    //...
    app.UseAuthentication();
    app.UseMvc();
}

Enforcer exception model

TODO: Describe the exception model returned

Extending enforcers

There is no way to actually extend the enforcers by adding new, but you can create a custom exception that implements the empty IEnforcerException interface. Throwing that custom exception will cause the enforcer middleware to catch the exception and return the serialized model of that.

Checks included

The checks don't throw anything! They just return false when something is wrong and true if check succeeds!

Bool Checks

IsTrue(bool value)
IsFalse(bool value)

Numeric Checks

AreEqual<T>(T obj1, T obj2) where T : IComparable
IsGreaterThan<T>(T value, T than) where T : IComparable
IsSmallerThan<T>(T value, T than) where T : IComparable
IsBetweenInclusive<T>(T value, T from, T to) where T : IComparable
IsBetweenExclusive<T>(T value, T from, T to) where T : IComparable

Object Checks

AreEqual<T>(T value1, T value2) where T : IComparable
IsNull<T>(T value)
IsNull<T>(T? value) where T : struct
IsDefault<T>(T value)
IsDefined<T>(T value) where T : struct

String Checks

IsNullOrEmpty(string value)
IsNullOrWhiteSpace(string value)
IsNull(string value)

georgekosmidis / Action.Guards