Modify Request.Content in WebApi DelegatingHandler

Question

I need to modify requested content to replace some characters (because of some unicode problems). Previously (in ASP.NET MVC), I did this with HttpModules; but in WebApi, it seems that I should DelegatingHandler but it is totally different.

How can I modify request.Content inside the SendAsync method? I need something like this:

protected async override Task<HttpResponseMessage> SendAsync(
        HttpRequestMessage request, CancellationToken cancellationToken)
    {
        var s = await request.Content.ReadAsStringAsync();
    // do some modification on "s"
    s= s.replace("x","y");

    request.Content = new StringContent(s);

    var response = await base.SendAsync(request, cancellationToken);
    
    return response;
}

In the code above, I think I should check the request's content type and then decide what to do. If yes, which checks should I do?

Pang · Accepted Answer · 2020-09-16 06:38:45Z

I did something like this in SendAsync. Although it is not a comprehensive solution, it works:

//first : correct the URI (querysting data) first
request.RequestUri = new Uri(Correcr(request.RequestUri.ToString()));

var contentType = request.Content.Headers.ContentType;

if (contentType != null)
{
    if (contentType.MediaType == "application/x-www-form-urlencoded")//post,put,... & other non-json requests
    {
        var formData = await request.Content.ReadAsFormDataAsync();
        request.Content = new FormUrlEncodedContent(Correct(formData));
    }
    else if (contentType.MediaType == "multipart/form-data")//file upload , so ignre it
    {
        var formData = await request.Content.ReadAsFormDataAsync();
        request.Content = new FormUrlEncodedContent(Correct(formData));
    }
    else if (contentType.MediaType == "application/json")//json request
    {
        var oldHeaders = request.Content.Headers;
        var formData = await request.Content.ReadAsStringAsync();
        request.Content = new StringContent(Correct(formData));
        ReplaceHeaders(request.Content.Headers, oldHeaders);
    }
    else
        throw new Exception("Implement It!");
}

return await base.SendAsync(request, cancellationToken);

and this helper function:

private void ReplaceHeaders(HttpContentHeaders currentHeaders, HttpContentHeaders oldHeaders)
{
    currentHeaders.Clear();
    foreach (var item in oldHeaders)
        currentHeaders.Add(item.Key, item.Value);
}

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