Problem with Content-Type in a (.net) StringContent

Question

I have a request (going to an APIM endpoint.) From Postman the test runs just fine, but from my code it fails with a 400. I looked at the requests in fiddler and the only difference is this:

 // This works
 Content-Type: application/json

 // This gives a 400
 Content-Type: application/json; charset=utf-8

APIM is supposed to use UTF-8 by default, so I don't understand. Plus there doesn't seem to be a way within the C# code to produce the first. If I try to add the header directly it complains that I need to add it as part of the content string. So I have this:

  requestMessage.Content = new StringContent(content, Encoding.UTF8, "application/json");

This produces the second Content-Type header. There seems to be no overload on the StringContent class to allow me to produce the first version (without specifying the encoding.)

I'm stuck. It seems a really simply problem but I don't see a way to get through all the "helper" code that Microsoft has put in place to get what I need.

Insofar as it matters I am running .net8.

Any suggestions?

How about if you clear the CharSet? content.Headers.ContentType.CharSet = null — Magnus
– Magnus, Commented Nov 4 at 8:12
What is APIM? What you describe sounds like a service bug. Also, why are you serializing manually instead of using eg PostAsJsonAsync ? Or use JsonContent ? JsonContent.Create allows you to set your own MediaTypeHeaderValue — Panagiotis Kanavos
– Panagiotis Kanavos, Commented Nov 5 at 13:38
@PanagiotisKanavos APIM is a component of Azure for creating APIs and I am not serializing manually, I am looking at the output the regular libraries produce. — Fraser Orr
– Fraser Orr, Commented Nov 5 at 14:19
What's the exact name? Almost certainly, it has a C# SDK. I am not serializing manually if you generate a string from your DTO and send it as a StringContent , you are. You can just post your DTO so why don't you? Was there a problem? — Panagiotis Kanavos
– Panagiotis Kanavos, Commented Nov 5 at 14:23

Austin Winstanley · Accepted Answer · 2025-11-05 12:28:22Z

1

Need to clear the headers and then set them:

var stringContent = new StringContent(content);
stringContent.Headers.Clear();
stringContent.Headers.ContentType = new MediaTypeHeaderValue(MediaTypeNames.Application.Json);

requestMessage.Content = stringContent;

Here is a unit test you can run debugger on that will return the result you want.

[Fact]
public void TestStringContentHeaders()
{
    var stringContent = new StringContent(string.Empty);
    stringContent.Headers.Clear();
    stringContent.Headers.ContentType = new MediaTypeHeaderValue(MediaTypeNames.Application.Json);
    Assert.Equal("application/json", stringContent.Headers.ContentType?.MediaType);
    Assert.Null(stringContent.Headers.ContentType?.CharSet);
}

answered Nov 5 at 12:28

Austin Winstanley

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Comments

Panagiotis Kanavos · Accepted Answer · 2025-11-05 14:32:14Z

It's unclear what APIM means, why that service complains or why the code uses manual serialization to a string. JSON must be UTF8 so charset has no effect and should be ignored by the recipient. RFC-8259 says :

Note: No "charset" parameter is defined for this registration. Adding one really has no effect on compliant recipients.

Instead of manually serializing to a string you can use JsonContent.Create with an explicit MediaTypeHeaderValue value that doesn't include charset.


var myDTO=...;
var jsonType=new MediaTypeHeaderValue("application/json");
requestMessage.Content = JsonContent.Create(myDTO, jsonType);

You don't have to clear the headers, you just pass the headers you actually want.

If APIM means the Azure API Management, there's a .NET SDK for it with a Quickstart in Github. Admittedly, a Quickstart isn't enough documentation. Using the SDK means you don't have to manually create DTOs and serialize them.

The Quickstart page shows how you can create a resource for example :

using System;
using System.Threading.Tasks;
using Azure.Core;
using Azure.Identity;
using Azure.ResourceManager;
using Azure.ResourceManager.Compute;
using Azure.ResourceManager.Resources;

// Code omitted for brevity

// First, initialize the ArmClient and get the default subscription
ArmClient client = new ArmClient(new DefaultAzureCredential());
// Now we get a ResourceGroupResource collection for that subscription
SubscriptionResource subscription = await client.GetDefaultSubscriptionAsync();
ResourceGroupCollection resourceGroups = subscription.GetResourceGroups();

// With the collection, we can create a new resource group with an specific name
string resourceGroupName = "myRgName";
AzureLocation location = AzureLocation.WestUS2;
ResourceGroupData resourceGroupData = new ResourceGroupData(location);
ArmOperation<ResourceGroupResource> operation = await resourceGroups.CreateOrUpdateAsync(WaitUntil.Completed, resourceGroupName, resourceGroupData);
ResourceGroupResource resourceGroup = operation.Value;

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