构建动态lambda表达式

Question

我知道如何构建一个像x => x> 5的简单lambda：

int[] nbs = new[] { 1, 2, 3, 4, 5, 6, 7, 8, 9, 10 };            
IEnumerable<int> result1 = nbs.Where(x => x > 5);

ParameterExpression parameter = Expression.Parameter(typeof(int), "x");
ConstantExpression constant = Expression.Constant(5);
BinaryExpression expressionBody = Expression.GreaterThan(parameter, constant);
Expression<Func<int, bool>> expression = Expression.Lambda<Func<int, bool>>(expressionBody, parameter);
IEnumerable<int> result2 = nbs.Where(expression.Compile());

但是我如何以与上面类似的方式构建像x => whiteNbs.Contains（x）这样的lambda：

int[] nbs = new[] { 1, 2, 3, 4, 5, 6, 7, 8, 9, 10 };
List<int> whiteNbs = new List<int>() { 1, 5 };            
IEnumerable<int> result = nbs.Where(x => whiteNbs.Contains(x));

Answer 1

用GreaterThan替换二进制MethodCallExpression表达式。

int[] nbs = new[] { 1, 2, 3, 4, 5, 6, 7, 8, 9, 10 };

var elementType = typeof(int);
var x = Expression.Parameter(elementType, "x");
var whiteNbs = Expression.Constant(new List<int>() {1, 5});

var contains = Expression.Call(typeof(Enumerable),
    "Contains",
    new[] {elementType},
    whiteNbs, 
    x
  );

var lambda = Expression.Lambda<Func<int, bool>>(contains, x);
var result = nbs.Where(lambda.Compile());