Get Primary Key of table using only string type table name in Linq

Hello,
I'm having trouble with obtaining the primary key using the LINQ datacontext and when I only have the table name in a string variable.  What I am trying is below, but it doesn't work because I can't get the Type argument of GetTable

I'm building a dynamic table editor using the GridView control and LINQDataSource as the datasource so I need to get the primary key of the table being edited so I can assign it to the GridView.DataKeyNames property.

Anyone run into this?

//This works because I write the actual entity name 'MainTable' in the code:
 
string pkName = _dc.Mapping.GetTable(typeof(MainTable)).RowType.IdentityMembers[0].Name;
 
//But I want to dynamically get the primary key of a table that is stored in a string variable.
//I have tried this:
 
Type tableType = Type.GetType("My.Namespace.." + _tableName);
pkName = _dc.Mapping.GetTable(typeof(tableType)).RowType.IdentityMembers[0].Name;
 
//This doesn't work because it wants an actual class name in the GetTable() method.

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