Question

First let me start by saying that I don’t have a complete understanding of Linq. I am trying to dynamically query a database, The first query uses LINQ-SQL which works fine, but the second dynamic call is what fails in run time

public void getTables()
    {
        foreach (var c in dc.GetTable<TableListing>())
        {
            List<TableData> res = tableBrowse(c.TableName);
        }                  
    }

 public List<TableData> tableBrowse(string tablename)
    {
        string sql = "Select * from " + tablename;
        var results = dc.ExecuteQuery<TableData>(sql);
        return results.ToList();
    }

public class TableData
    {
        public int Time { get; set; }
        public string Value { get; set; }
    }

I query the “master table” and this retrieves a list of tables to query. They all have the same structure, as defined in the class TableData. I get a runtime error about Specified cast is not valid. I’m not really looking for code as much as I am looking for what I am doing wrong and how to fix it. Thanks.

Answer 1

You aren’t explicitly converting the return value from dc.ExecuteQuery<TableData>(sql) to the TableData type that you’ve defined. I expect that the ExecuteQuery is complaining because it doesn’t know what the TableData type is.

The ExecuteQuery helper needs to return a DBML (LINQ-to-SQL generated) type as defined in your database.

But I would suggest that you don’t go down this route. If you want to get records from a table, say Customers, just use content.Customers – the point of LINQ-to-SQL is that it already contains all these accessors to save you time.