How to set variables in the API request pipeline

June 25, 2020 2-minute read

.NET

The code for this example can be found on Github

Often you want to pass variables from claims or headers into your application. Instead of setting these variables in your controller you can add a middleware that stores variables in the HttpContext.Items collection and this collection is accessible inside a controller. To make it cleaner here I added an abstract controller that makes the variables available to the values controller.

RequestVariablesMiddleware.cs

using Microsoft.AspNetCore.Builder;
using Microsoft.AspNetCore.Http;
using System.Threading.Tasks;

namespace RequestVariableMiddleware
{
    public class VariableMiddleware
    {
        private readonly RequestDelegate next;

        public VariableMiddleware(RequestDelegate next)
        {
            this.next = next;
        }

        public async Task Invoke(HttpContext httpContext)
        {
            RequestVariables variables = new RequestVariables();
            variables.ApiKey = httpContext.Request.Headers["Authorization"];
            httpContext.Items["Variables"] = variables;
            await this.next(httpContext);
        }
    }

    public static class VariableMiddlewareExtensions
    {
        public static IApplicationBuilder UseVariableMiddleware(
            this IApplicationBuilder builder)
        {
            return builder.UseMiddleware<VariableMiddleware>();
        }
    }
}

Startup.cs

public void Configure(IApplicationBuilder app, IWebHostEnvironment env)
{
    if (env.IsDevelopment())
    {
        app.UseDeveloperExceptionPage();
    }

    app.UseHttpsRedirection();

    app.UseVariableMiddleware();

    ...

AbstractController.cs

using System;
using Microsoft.AspNetCore.Mvc;

namespace RequestVariableMiddleware.Controllers
{
    [Route("api/[controller]")]
    [ApiController]
    public abstract class AbstractController : ControllerBase
    {
        public RequestVariables RequestVariables 
        {
            get
            {
                RequestVariables variables = this.HttpContext.Items["Variables"] as RequestVariables;
                if(variables == null)
                {
                    throw new NullReferenceException("Variables not set in the request pipeline.");
                }
                else
                {
                    return variables;
                }
            }
        }
    }
}

ValuesController.cs

using Microsoft.AspNetCore.Mvc;

namespace RequestVariableMiddleware.Controllers
{
    [Route("api/[controller]")]
    [ApiController]
    public class ValuesController : AbstractController
    {

        public string Get()
        {
            return $"API_KEY={this.RequestVariables.ApiKey}";
        }
    }
}