Question

I get InvalidCastException when I call First() at the last line. Can anyone tell me why?

static bool TableExists(string tableNameAndSchema) 
{
    string checkTable =
        String.Format(
        "IF OBJECT_ID('{0}', 'U') IS NOT NULL SELECT 'true' ELSE SELECT 'false'",
        tableNameAndSchema);            

    var result = db.ExecuteQuery<bool>(checkTable);
    return result.First();
}

Answer 1

Both of the following two formats work properly. The core of the challenge appears to be what type is returned by ExecuteQuery.

SELECT ‘true’ actually returns a string and not a bool – thus the ExecuteQuery result variable also has to be string (and not bool) if using that format

And when using SELECT 1 as suggested by @Eben Roux and @Anders Abel, then the ExecuteQuery result variable has to be int (and not bool)

static bool TableExists(string tableNameAndSchema)
{
    string checkTable =
        String.Format(
        "IF OBJECT_ID('{0}', 'U') IS NOT NULL SELECT 'true' ELSE SELECT 'false'",
        tableNameAndSchema);

    IEnumerable<string> result = db.ExecuteQuery<string>(checkTable);
    return Convert.ToBoolean(result.First());
}

static bool TableExists2(string tableNameAndSchema)
{
    string checkTable =
        String.Format(
        "IF OBJECT_ID('{0}', 'U') IS NOT NULL SELECT 1 ELSE SELECT 0",
        tableNameAndSchema);

    IEnumerable<int> result = db.ExecuteQuery<int>(checkTable);
    return Convert.ToBoolean(result.First());
}

Thanks to @Eben Roux and @Anders Abel whose answers pointed me toward the final solution(s) – (I marked your answers as useful as they steered me in the right direction to get the final answer – thanks guys 🙂 )