Outsmarting the C# Compiler - Using Static Types as Type Arguments

Bernhard Speiser — Thu, 16 Sep 2021 14:48:45 +0000

In case you were not aware, the C# compiler prevents you from using static types as type arguments. Trying to use a static type will result in the compiler error CS0718.

error CS0718:
  'StaticClass': static types cannot be used as type arguments

This can be demonstrated by the following example:

static class StaticClass { }
static void Do<T>() => Console.WriteLine(typeof(T).FullName);

Do<StaticClass>();

Outsmarting the Compiler

We need to move the Do<T> method into a new class and instantiate an instance of that class.

class MyClass
{
    public void Do<T>() => Console.WriteLine(typeof(T).FullName);
}

var instance = new MyClass();

This alone will not "fix" the issue, as calling instance.Do<StaticClass>() still results in the same compiler error.

This is where dynamic comes into play.

((dynamic)instance).Do<StaticClass>();

Since instance is now of type dynamic the compiler will not verify the Do call at compile time, but at runtime. Now the code builds. Running it will print StaticClass to the console, which indicates that only the compiler prevents a type argument from being static, although the runtime can handle static types as type arguments.

The need to outsmart the compiler, in most cases, indicates that there is something wrong with your design. Remember, just because you can, doesn't mean you should.

Building an Express Gateway Policy

Bernhard Speiser — Sun, 29 Aug 2021 08:36:21 +0000

This post will show you how to build a policy (middleware) for your express gateway. Before creating a policy, we need to create a plugin.

Creating a plugin

To create a plugin, we need to add a folder plugins/my-plugin for it and a manifest file plugins/my-plugin/manifest.js with the following content

// plugins/my-plugin/manifest.js
module.exports = {
  version: '0.0.1',
  schema: {
    "$id": "https://express-gateway.io/schemas/plugins/my-plugin.json"
  }
}

Once this is done, we need to register the plugin in the system.config.yml file. Depending on your file structure, the file path may contain more or less ../ than this example.

# system.config.yml

plugins:
+  my-plugin:
+    package: '../../../plugins/my-plugin/manifest.js'

Creating the policy

Now that we have a plugin, we can start creating our policy. Add a policies folder plugins/my-plugin/policies and the policy itself plugins/my-plugin/policies/my-policy.js with the following content

// plugins/my-plugin/policies/my-policy.js
module.exports = {
  name: 'my-policy',
  schema: {
    $id: 'http://express-gateway.io/schemas/policies/my-policy.json',
    type: 'object'
  },
  policy: (actionParams) => {
    return (req, res, next) => {
      // TODO: Implement me
      console.log('hello world');
      next();
    };
  }
};

The policy function will be the place where you implement your specific use case.

Now we need to register the policy as part of the plugin

// plugins/my-plugin/manifest.js
module.exports = {
  version: '0.0.1',
+ init: function (pluginContext) {
+   let policy = require('./policies/my-policy')
+   pluginContext.registerPolicy(policy)
+ },
+ policies: ['my-policy'],
  schema: {
    "$id": "https://express-gateway.io/schemas/plugins/my-plugin.json"
  }
}

Using the policy

Finally, we can add the policy as any other default policy in the gateway.config.yml

# gateway.config.yml

policies:
  - log
  - jwt
+ - my-plugin

and use it in our pipelines

# gateway.config.yml

pipelines:
  my-pipeline:
    policies:
+     - my-plugin:
      - log:
        - action:
            message: ${req.method} ${req.originalUrl}

Now we are finished with our policy, but we can still enhance it a bit.

Adding parameters

In the case, that you want to configure your policy (without changing the code) you can specify parameters. The syntax used is called Json Schema. You can have a look at their documentation for the type that you need. In this case, we add one parameter myarray with the type string[]

// plugins/my-plugin/policies/my-policy.js

  schema: {
    $id: 'http://express-gateway.io/schemas/policies/my-policy.json',
-   type: 'object'
+   type: 'object',
+   properties: {
+     myarray: {
+       "type": "array",
+       "items": {
+         "type": "string"
+       }
+     }
+   }
  },

After that, we can pass our value from the pipeline

# gateway.config.yml

    policies:
     - my-plugin:
+       - action:
+           myarray: ['value1', 'value2']

and use it within the policy function: actionParams.myarray.

I hope this helped you understand how to set up a policy in an express gateway pipeline.

DEV Community: Bernhard Speiser