Salesforce REST Exceptions

I get a lot of emails from people working with the Salesforce REST who say that they are getting a 400 error (Bad Request) when trying to perform one of the API functions. In this article I will talk about this error and how to troubleshoot it as well as other API errors.
For the purpose of this article I will assume you are using an HttpWebRequest object to make your requests to Salesforce. The function on HttpWebRequest that does the actual work is  GetResponse. This function makes the call to the remote server and retrieves the response. If anything goes wrong with a REST API request on the Salesforce server it will generate an 400 HTTP error response code and then GetResponse will throw a WebException containing that status code. The 400 response code isn’t too useful since there are a lot of things that can cause it, but if you catch the exception you can still access the response stream which will contain a JSON object with more details.
As an example, if we make a call to get an Access Token and pass an incorrect client_secret you will get this web exception:
"The remote server returned an error: (400) Bad Request."
If we then get the response stream we find this:
{\"error\":\"invalid_client\",\"error_description\":\"invalid client credentials\"}
Here is another example. If we issue a query command with a syntax error in the SOQL query (SELEC id,accountNumber,name from account), you will get this:
{ \"message\" : \"unexpected token: SELEC\",\n  \"errorCode\" : \"MALFORMED_QUERY\"}
You can see that it didn’t recognize “SELEC” since it is missing the ‘T’. Note that the format of the JSON in this message is different then the first one.
So what is the best way to handle this? There are a number of options depending on you needs, but here is one suggestion. Here is the piece of code that makes the web request that would be in a library used by all Salesforce REST calls. ‘req’ is  a HttpWebRequest object.

req.Method = "GET";
System.Net.WebResponse resp = null;
try
{
    resp = req.GetResponse();
}
catch (WebException ex)
{
    string msg = "";

    if (ex.Status == WebExceptionStatus.ProtocolError)
    {
        resp = ex.Response;
        msg = new System.IO.StreamReader(resp.GetResponseStream()).ReadToEnd().Trim();
    }

    throw new SalesforceException(msg, ex);
}

In the try block we make the call to Salesforce and get the response. Next we catch only WebExceptions, other exceptions are allow to percolate up to the next level which is a good practice for library functions. Next we check that status of the WebException. There are a lot of things that can cause WebExceptions but most of them will result in a response not being returned, so we just look for a ProtocolError. If we do have a protocol error we retrieve the JSON error message from the response. Finally we create and throw a custom exception I call SalesforceException that will hold both the JSON message and the original exception in the InnerException. The code for the SalesforceExpcetion is here:

public class SalesforceException : Exception 
{
    public SalesforceException(string message, Exception innerException)
        : base(message, innerException) { }
}

One other thing to note about Salesforce REST errors. Since REST commands are based on URIs, if you have an error in the URI you will normally get back a HTTP 404 error instead of a 400 error. For example if we send a read command with an invalid object ID in the URI, we will get a 404 error instead of a 400 error.