C# LINQ to SQL: Refactoring this Generic GetByID method

What you need is to build an expression tree that LINQ to SQL can understand. Assuming your “id” property is always named “id”:

public virtual T GetById<T>(short id)
{
    var itemParameter = Expression.Parameter(typeof(T), "item");
    var whereExpression = Expression.Lambda<Func<T, bool>>
        (
        Expression.Equal(
            Expression.Property(
                itemParameter,
                "id"
                ),
            Expression.Constant(id)
            ),
        new[] { itemParameter }
        );
    var table = DB.GetTable<T>();
    return table.Where(whereExpression).Single();
}

This should do the trick. It was shamelessly borrowed from this blog.
This is basically what LINQ to SQL does when you write a query like

var Q = from t in Context.GetTable<T)()
        where t.id == id
        select t;

You just do the work for LTS because the compiler cannot create that for you, since nothing can enforce that T has an “id” property, and you cannot map an arbitrary “id” property from an interface to the database.

==== UPDATE ====

OK, here’s a simple implementation for finding the primary key name, assuming there is only one (not a composite primary key), and assuming all is well type-wise (that is, your primary key is compatible with the “short” type you use in the GetById function):

public virtual T GetById<T>(short id)
{
    var itemParameter = Expression.Parameter(typeof(T), "item");
    var whereExpression = Expression.Lambda<Func<T, bool>>
        (
        Expression.Equal(
            Expression.Property(
                itemParameter,
                GetPrimaryKeyName<T>()
                ),
            Expression.Constant(id)
            ),
        new[] { itemParameter }
        );
    var table = DB.GetTable<T>();
    return table.Where(whereExpression).Single();
}


public string GetPrimaryKeyName<T>()
{
    var type = Mapping.GetMetaType(typeof(T));

    var PK = (from m in type.DataMembers
              where m.IsPrimaryKey
              select m).Single();
    return PK.Name;
}