How do I pass an object to HttpClient.PostAsync and serialize as a JSON body?

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The straight up answer to your question is: No. The signature for the PostAsync method is as follows:

public Task PostAsync(Uri requestUri, HttpContent content)

So, while you can pass an object to PostAsync it must be of type HttpContent and your anonymous type does not meet that criteria.

However, there are ways to accomplish what you want to accomplish. First, you will need to serialize your anonymous type to JSON, the most common tool for this is Json.NET. And the code for this is pretty trivial:

var myContent = JsonConvert.SerializeObject(data);

Next, you will need to construct a content object to send this data, I will use a ByteArrayContent object, but you could use or create a different type if you wanted.

var buffer = System.Text.Encoding.UTF8.GetBytes(myContent);
var byteContent = new ByteArrayContent(buffer);

Next, you want to set the content type to let the API know this is JSON.

byteContent.Headers.ContentType = new MediaTypeHeaderValue("application/json");

Then you can send your request very similar to your previous example with the form content:

var result = client.PostAsync("", byteContent).Result

On a side note, calling the .Result property like you’re doing here can have some bad side effects such as dead locking, so you want to be careful with this.

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