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Related Code

For usage see Queryable.Join reference: http://msdn.microsoft.com/en-us/library/bb534644(v=vs.95).aspx.

It's uses a help class (JoinTuple<,>) for the groupjoin.

public static IQueryable<TResult> LeftOuterJoin<TOuter, TInner, TKey, TResult>(
    this IQueryable<TOuter> outer,
    IQueryable<TInner> inner,
    Expression<Func<TOuter, TKey>> outerKeySelector,
    Expression<Func<TInner, TKey>> innerKeySelector,
    Expression<Func<TOuter, TInner, TResult>> resultSelector)
{

    System.Type t1 = typeof(TOuter);
    System.Type t2 = typeof(IEnumerable<TInner>);
    System.Type groupJoinType = typeof(JoinTuple<TOuter, IEnumerable<TInner>>);
    var param1 = Expression.Parameter(t1, "w");
    var param2 = Expression.Parameter(t2, "f");
    var groupJoinMemberInitLambda = Expression.Lambda(
    	Expression.MemberInit(
    		Expression.New(groupJoinType),
    		Expression.Bind(groupJoinType.GetProperty("Item1"), param1),
    		Expression.Bind(groupJoinType.GetProperty("Item2"), param2)
    	), param1, param2);

    var q = outer.Provider.CreateQuery(
    	Expression.Call(
    		typeof(Queryable),
    		"GroupJoin",
    		new System.Type[]
    		{
    			typeof(TInner),
    			typeof(TOuter),
    			typeof(TKey),
    			groupJoinType
    		},
    		outer.Expression,
    		inner.Expression,
    		outerKeySelector,
    		innerKeySelector,
    		Expression.Quote(groupJoinMemberInitLambda)));

    ParameterExpression defaultIfParameter = Expression.Parameter(groupJoinType, "f");
    MethodCallExpression defaultIfEmptyExpression2 =
    	Expression.Call(
    		typeof(Enumerable),
    		"DefaultIfEmpty",
    		new System.Type[] { typeof(TInner) },
    		Expression.Property(defaultIfParameter, "Item2"));
    LambdaExpression defaultIfEmptyLambda2 = Expression.Lambda(defaultIfEmptyExpression2, defaultIfParameter);

    var p1 = Expression.Parameter(groupJoinType, "t");
    var p2 = Expression.Parameter(typeof(TOuter), "o");
    var resultInvokeExpression = Expression.Invoke(resultSelector, Expression.Property(p1, "Item1"), p2);

    return q.Provider.CreateQuery(
    	Expression.Call(
    		typeof(Queryable),
    		"SelectMany",
    		new System.Type[]
    		{
    			groupJoinType,
    			typeof(TInner),
    			typeof(TResult)
    		},
    		q.Expression,
    		defaultIfEmptyLambda2,
    		Expression.Lambda(
    			resultInvokeExpression,
    			p1,
    			p2))) as IQueryable<TResult>;
}


public class JoinTuple<T1, T2>
{
    public T1 Item1 { get; set; }
    public T2 Item2 { get; set; }
}

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