Linq IN operator

Sometimes, you might want to extract the "intersection" of the two object lists but the object types are different. For example, you have a customer list and another one is invoice list. You want to get the customers whose "customer_id" exist in the invoice list. This can be done in the SQL syntax easily by using the IN operator. But, there is none in the Linq. So, I have written my own extension:

public static List<T> In<T, T2>(this List<T> list1,
           List<T2> list2,
           Func<T, T2, bool> predicate)
        {
            List<T> result = new List<T>();

           // exit immediately if list2 is empty or null.
            if ((list2 == null)
                || (list2.Count == 0))
            {
                return result;     // return blank list.
            }

            foreach (var item in list1)
            {
                if (list2.Where(n => predicate(item, n)).Count() > 0)
                {
                    result.Add(item);
                }
            }

            return result;
        }

Dynamic sorting using Linq Expression

You may wonder how to avoid hard coding the sorting order in the list. Here is one of the solution using Linq Expression:

class MyClass
{
   public int age { get; set; }
   public string name { get; set; }
}

MyClass[] arr =
{
   new MyClass() {age = 10, name ="abc" },
   new MyClass() {age = 20, name ="xyz" },
   new MyClass() {age = 30, name ="cde" }
};


// create the parameter
ParameterExpression param = Expression.Parameter(typeof(MyClass), "i");

// create the property to be used base on the 
// type (ie, MyClass) in the param variable. 
// you may change the "name" to any other field.
Expression prop = Expression.Property(param, "name");

// construct the lambda expression.
Expression> lambda 
   = Expression.Lambda>(prop, param);

// compile it to a function.
Func compile = lambda.Compile();

// now, we can use the function by passing it 
// to the OrderBy() proc.
var v3 = arr.OrderBy(compile);

string s;
foreach (var item in v3)
{
   s = item.name;
}

Linq literals

Check this out if you are passing the string into Where():

http://msdn.microsoft.com/en-us/library/bb399176.aspx