Update wiki page of tfs by calling rest-api - rest

Question

I want to update an already existing wiki page of tfs by using this document:
https://learn.microsoft.com/de-de/rest/api/azure/devops/wiki/pages/create%20or%20update?view=azure-devops-rest-4.1
Creating a new wiki-page (with content) is no problem. That is working fine.
But I want to edit an existing one. The tfs-documentation says that the only difference in API call is to use an "If-Match"-header (see section Request Header).
Here I have 3 situations:
Using no "If-Match"-Header or an empty: Get a "412 Precondition Failed" error.
Using a "If-Match"-Header with random value: Get a "400 Bad Request" error.
Using a "If-Match"-Header with exactly 40 characters (like the version-hash of the page-revision (e.g. '09f62be600a3b6d36d21b294dbb00921a5ba03ec')): Again "412 Precondition Failed" error.
I think the revision-hash (40 characters) should be a good way because the error message on non-40-chars returns the 400-error.
But it did not work? Has anyone an idea which id tfs is wanting? I used Postman and C# to update by API. Below you can see my example code:
var handler = new HttpClientHandler()
{
UseDefaultCredentials = true,
UseProxy = false,
};
var client = new HttpClient(handler);
client.BaseAddress = new Uri(".../pages/pagename" + "?api-version=4.1");
client.DefaultRequestHeaders.Accept.Add(new System.Net.Http.Headers.MediaTypeWithQualityHeaderValue("application/json"));
var putContent = new StringContent("{ \"content\": \"New content for page\" }", Encoding.UTF8, "application/json");
client.DefaultRequestHeaders.TryAddWithoutValidation("If-Match", "09f62be600a3b6d36d21b294dbb00921a5ba03ec");
var responseTask = client.PutAsync(client.BaseAddress, putContent);
var result = responseTask.Result;
var content = result.Content.ReadAsStringAsync().Result;
var code = result.StatusCode;
var body = content;

Answer 1

According to the Create or Update Wiki API, if we want to edit the wiki page,If-Match header is required. The value of If-Matchth is the wiki page ETag.
ETags can also be used for optimistic concurrency control, as a way to help prevent simultaneous updates of a resource from overwriting each other
so we need to get the wiki Etag before update. Please have a try to change the code as following:
var baseUrl = "xxxxx";
var handler = new HttpClientHandler()
{
UseDefaultCredentials = true,
UseProxy = false,
};
var client = new HttpClient(handler)
{
BaseAddress = new Uri(baseUrl)
};
client.DefaultRequestHeaders.Accept.Add(new MediaTypeWithQualityHeaderValue("application/json"));
client.DefaultRequestHeaders.Authorization = new AuthenticationHeaderValue("Basic", "YourToken");
var getResult = client.GetAsync(baseUrl).Result;
var etag = getResult.Headers.GetValues("ETag");
var putContent = new StringContent("{ \"content\": \"New content for page\" }", Encoding.UTF8, "application/json");
client.DefaultRequestHeaders.TryAddWithoutValidation("If-Match", etag);
var responseTask = client.PutAsync(client.BaseAddress, putContent);
var result = responseTask.Result;
var content = result.Content.ReadAsStringAsync().Result;
Test Result: