SharePoint 2013 Search Rest POST error __metadata does not exist

Question

I am attempting a post request, and I have the following body:

{
    "request": {
        "__metadata": {
            "type": "Microsoft.Office.Server.Search.REST.SearchRequest"
        },
        "Querytext": "query",
        "TrimDuplicates": false,
        "RowLimit": 100,
        "SelectProperties": {
            "results": [
                "Title"
            ]
        },
        "SortList": {
            "results": [
                {
                    "Property": "Title",
                    "Direction": "0"
                }
            ]
        },
        "SourceId": "d2045d82-3148-4a4d-9dfc-fecb81da2720"
    }
}

I get the following response:

{"error":{"code":"-1, Microsoft.Data.OData.ODataException","message":{"lang":"en-US","value":"The property '__metadata' does not exist on type 'Microsoft.Office.Server.Search.REST.SearchRequest'. Make sure to only use property names that are defined by the type."}}}

I have found this answer, however I already have my body formatted in the way the answer suggests.

Answer 1

Turned out to be an issue with RestSharp, and it overwriting the Content-Type header. The following resolved the issue:

var request = new RestRequest
{
    Method = Method.POST
};
request.Parameters.Clear();
request.AddParameter(
    "application/json;odata=verbose",
    javaScriptSerializer.Serialize( requestBody ),
    ParameterType.RequestBody );

Doing it the normal way sets the Content-Type to "application/json", which causes the error I was getting.