string tree1 = "maple";
string tree2 = "oak";

System.Reflection.MethodInfo addMethod = typeof(Dictionary<int, string>).GetMethod("Add");

// Create two ElementInit objects that represent the
// two key-value pairs to add to the Dictionary.
System.Linq.Expressions.ElementInit elementInit1 =
    System.Linq.Expressions.Expression.ElementInit(
        addMethod,
        System.Linq.Expressions.Expression.Constant(tree1.Length),
        System.Linq.Expressions.Expression.Constant(tree1));
System.Linq.Expressions.ElementInit elementInit2 =
    System.Linq.Expressions.Expression.ElementInit(
        addMethod,
        System.Linq.Expressions.Expression.Constant(tree2.Length),
        System.Linq.Expressions.Expression.Constant(tree2));

// Create a NewExpression that represents constructing
// a new instance of Dictionary<int, string>.
System.Linq.Expressions.NewExpression newDictionaryExpression =
    System.Linq.Expressions.Expression.New(typeof(Dictionary<int, string>));

// Create a ListInitExpression that represents initializing
// a new Dictionary<> instance with two key-value pairs.
System.Linq.Expressions.ListInitExpression listInitExpression =
    System.Linq.Expressions.Expression.ListInit(
        newDictionaryExpression,
        elementInit1,
        elementInit2);

Console.WriteLine(listInitExpression.ToString());