Remove instanceId from query string value and return a new query string. If instanceId is not present, return the original query string.

RemoveInstanceId.cs

// NOTE: assumption that query doesn't start with '?'
static string? RemoveInstanceIdFromQueryString(string query)
{
    if (string.IsNullOrWhiteSpace(query))
    {
        return query;
    }
    
    // Opting to not use a StringBuilder as a single string concat feels OK re: allocations
    
    // First check if it's the first entry in the query string, and if not it'll be &instanceId
    if (query.StartsWith("instanceId"))
    {
        // It may be the entire query string in which case we return empty string
        var nextParamLocation = query.IndexOf('&');
        if (nextParamLocation == -1)
        {
            return "";
        }
        return query.Substring(nextParamLocation + 1);
    }
    
    // If we get here it either isn't in the query string at all or it's part of the query string
    // somewhere that isn't the beginning so it'll appear as &instanceId
    
    var locationIndex = query.IndexOf("&instanceId");
    if (locationIndex == -1)
    {
        // It doesn't exist
        return query;
    }
    
    // Check for it being the very last param
    var followingParamIndex = query.IndexOf('&', locationIndex + 1);
    if (followingParamIndex == -1)
    {
        return query.Substring(0, locationIndex);
    }
    
    return query.Substring(0, locationIndex) + query.Substring(followingParamIndex);
}