设置响应ContentType的中间件 [英] Middleware to set response ContentType

问题描述

在基于ASP.NET Core的Web应用程序中，我们需要执行以下操作:某些请求的文件类型应获取自定义ContentType作为响应.例如. .map应该映射到application/json.在完整" ASP.NET 4.x中以及与IIS结合使用时，可以使用web.config <staticContent>/<mimeMap>来实现此功能，我想用自定义ASP.NET Core中间件来替换此行为.

In our ASP.NET Core based web application, we want the following: certain requested file types should get custom ContentType's in response. E.g. .map should map to application/json. In "full" ASP.NET 4.x and in combination with IIS it was possible to utilize web.config <staticContent>/<mimeMap> for this and I want to replace this behavior with a custom ASP.NET Core middleware.

因此，我尝试了以下操作(为简便起见而简化):

So I tried the following (simplified for brevity):

public async Task Invoke(HttpContext context)
{
    await nextMiddleware.Invoke(context);

    if (context.Response.StatusCode == (int)HttpStatusCode.OK)
    {
        if (context.Request.Path.Value.EndsWith(".map"))
        {
            context.Response.ContentType = "application/json";
        }
    }
}

不幸的是，在调用中间件链的其余部分之后尝试设置context.Response.ContentType会导致以下异常:

Unfortunately, trying to set context.Response.ContentType after invoking the rest of the middleware chain yields to the following exception:

System.InvalidOperationException: "Headers are read-only, response has already started."

如何创建可满足此要求的中间件?

How can I create a middleware that solves this requirement?

推荐答案

尝试使用HttpContext.Response.OnStarting回调.这是发送标头之前触发的最后一个事件.

Try to use HttpContext.Response.OnStarting callback. This is the last event that is fired before the headers are sent.

public async Task Invoke(HttpContext context)
{
    context.Response.OnStarting((state) =>
    {
        if (context.Response.StatusCode == (int)HttpStatusCode.OK)
        {
           if (context.Request.Path.Value.EndsWith(".map"))
           {
             context.Response.ContentType = "application/json";
           }
        }          
        return Task.FromResult(0);
    }, null);

    await nextMiddleware.Invoke(context);
}

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