<Pre>no mapping exists from object type system.int32[] to a known managed provider native type.</pre>

You are trying to insert an integer array into an int database field.
You should loop through your array and insert each single integer value.

Let's break down the error:

No mapping

So we're dealing with some conversion between values.

System.Int32[]

So the source data is of type int32[] which would be diagnostic_data given your code.

known managed provider native type.

A type that the SQL adapter (the provider) you're using knows about for the database. As in, it doesn't know of a database type that it can use in this situation.

So putting it all together: int32[] has no native database type it can convert into. This makes complete sense. When you use AddWithValue, you're creating an SqlParameter[^]. An SqlParameter is treated as a single literal value for safety reasons (sql injection). Since an array of integers has no obvious single literal value it can convert into, you get the error.

So how to fix it? I rather like this solution[^] found right here on CodeProject. You could also manually build an array of SqlParameters, something like:

SqlCommand cmd = new SqlCommand("INSERT INTO Table_1 VALUES(@value0, @value1)"); /*add however many values you need*/
List<SqlParameter> commands = new List<SqlParameter> {
    new SqlParameter("@value0", diagnostic_data[0]),
    new SqlParameter("@value1", diagnostic_data[1])
    //etc..
};
cmd.Parameters.AddRange(commands);

To my knowledge though, there's no way to do this with a single parameter due to the way parameters are handled for safety.