Grafting LINQ onto C# 2 library
- by P Daddy
I'm writing a data access layer. It will have C# 2 and C# 3 clients, so I'm compiling against the 2.0 framework. Although encouraging the use of stored procedures, I'm still trying to provide a fairly complete ability to perform ad-hoc queries. I have this working fairly well, already.
For the convenience of C# 3 clients, I'm trying to provide as much compatibility with LINQ query syntax as I can. Jon Skeet noticed that LINQ query expressions are duck typed, so I don't have to have an IQueryable and IQueryProvider (or IEnumerable<T>) to use them. I just have to provide methods with the correct signatures.
So I got Select, Where, OrderBy, OrderByDescending, ThenBy, and ThenByDescending working. Where I need help are with Join and GroupJoin. I've got them working, but only for one join.
A brief compilable example of what I have is this:
// .NET 2.0 doesn't define the Func<...> delegates, so let's define some workalikes
delegate TResult FakeFunc<T, TResult>(T arg);
delegate TResult FakeFunc<T1, T2, TResult>(T1 arg1, T2 arg2);
abstract class Projection{
public static Condition operator==(Projection a, Projection b){
return new EqualsCondition(a, b);
}
public static Condition operator!=(Projection a, Projection b){
throw new NotImplementedException();
}
}
class ColumnProjection : Projection{
readonly Table table;
readonly string columnName;
public ColumnProjection(Table table, string columnName){
this.table = table;
this.columnName = columnName;
}
}
abstract class Condition{}
class EqualsCondition : Condition{
readonly Projection a;
readonly Projection b;
public EqualsCondition(Projection a, Projection b){
this.a = a;
this.b = b;
}
}
class TableView{
readonly Table table;
readonly Projection[] projections;
public TableView(Table table, Projection[] projections){
this.table = table;
this.projections = projections;
}
}
class Table{
public Projection this[string columnName]{
get{return new ColumnProjection(this, columnName);}
}
public TableView Select(params Projection[] projections){
return new TableView(this, projections);
}
public TableView Select(FakeFunc<Table, Projection[]> projections){
return new TableView(this, projections(this));
}
public Table Join(Table other, Condition condition){
return new JoinedTable(this, other, condition);
}
public TableView Join(Table inner,
FakeFunc<Table, Projection> outerKeySelector,
FakeFunc<Table, Projection> innerKeySelector,
FakeFunc<Table, Table, Projection[]> resultSelector){
Table join = new JoinedTable(this, inner,
new EqualsCondition(outerKeySelector(this), innerKeySelector(inner)));
return join.Select(resultSelector(this, inner));
}
}
class JoinedTable : Table{
readonly Table left;
readonly Table right;
readonly Condition condition;
public JoinedTable(Table left, Table right, Condition condition){
this.left = left;
this.right = right;
this.condition = condition;
}
}
This allows me to use a fairly decent syntax in C# 2:
Table table1 = new Table();
Table table2 = new Table();
TableView result =
table1
.Join(table2, table1["ID"] == table2["ID"])
.Select(table1["ID"], table2["Description"]);
But an even nicer syntax in C# 3:
TableView result =
from t1 in table1
join t2 in table2 on t1["ID"] equals t2["ID"]
select new[]{t1["ID"], t2["Description"]};
This works well and gives me identical results to the first case. The problem is if I want to join in a third table.
TableView result =
from t1 in table1
join t2 in table2 on t1["ID"] equals t2["ID"]
join t3 in table3 on t1["ID"] equals t3["ID"]
select new[]{t1["ID"], t2["Description"], t3["Foo"]};
Now I get an error (Cannot implicitly convert type 'AnonymousType#1' to 'Projection[]'), presumably because the second join is trying to join the third table to an anonymous type containing the first two tables. This anonymous type, of course, doesn't have a Join method.
Any hints on how I can do this?
