如何创建像System.Linq.Expressions.Expression？ [英] How to create a System.Linq.Expressions.Expression for Like?

问题描述

我创建了一个过滤的BindingList的>。它的伟大工程：

I created a filterable BindingList from this source. It works great:

list.Filter("Customer == 'Name'");

做什么它应该。该内部工作就像一个分析器，可以转换表达 == 或！= 到系统.Linq.Expressions.Expression 。在这种情况下， == 变成 System.Linq.Expressions.Expression.Equal 。

does what it should. The internals work like a parser, that converts the expression == or != into System.Linq.Expressions.Expression. In this case, == becomes System.Linq.Expressions.Expression.Equal.

不幸的是 System.Linq.Expressions.Expression 不包含运营商一样，我不知道如何解决这个问题。

Unfortunately System.Linq.Expressions.Expression does not contain a like operator and I don't know how to solve this.

最初的代码如下所示：

private static Dictionary<String, Func<Expression, Expression, Expression>> 
    binaryOpFactory = new Dictionary<String, Func<Expression, Expression, Expression>>();

static Init() {
    binaryOpFactory.Add("==", Expression.Equal);
    binaryOpFactory.Add(">", Expression.GreaterThan);
    binaryOpFactory.Add("<", Expression.LessThan);
    binaryOpFactory.Add(">=", Expression.GreaterThanOrEqual);
    binaryOpFactory.Add("<=", Expression.LessThanOrEqual);
    binaryOpFactory.Add("!=", Expression.NotEqual);
    binaryOpFactory.Add("&&", Expression.And);
    binaryOpFactory.Add("||", Expression.Or);
}

然后，我创建了一个会做我想做的表达式：

Then I created an expression that will do what I want:

private static System.Linq.Expressions.Expression<Func<String, String, bool>>
    Like_Lambda = (item, search) => item.ToLower().Contains(search.ToLower());

private static Func<String, String, bool> Like = Like_Lambda.Compile();

例如

e.g.

Console.WriteLine(like("McDonalds", "donAld")); // true
Console.WriteLine(like("McDonalds", "King"));   // false

但 binaryOpFactory 需要此：

Func<Expression, Expression, Expression>

预定义的表情似乎正是：

The predefined expressions seem to be exactly that:

System.Linq.Expressions.Expression.Or;

谁能告诉我怎么我的表情转换？

Can anyone tell me how to convert my expression?

推荐答案

是这样的：

static IEnumerable<T> WhereLike<T>(
        this IEnumerable<T> data,
        string propertyOrFieldName,
        string value)
{
    var param = Expression.Parameter(typeof(T), "x");
    var body = Expression.Call(
        typeof(Program).GetMethod("Like",
            BindingFlags.Static | BindingFlags.NonPublic | BindingFlags.Public),
            Expression.PropertyOrField(param, propertyOrFieldName),
            Expression.Constant(value, typeof(string)));
    var lambda = Expression.Lambda<Func<T, bool>>(body, param);
    return data.Where(lambda.Compile());
}
static bool Like(string a, string b) {
    return a.Contains(b); // just for illustration
}

---

---

在 Func键和LT而言，表达，表达，表达> ：

static Expression Like(Expression lhs, Expression rhs)
{
    return Expression.Call(
        typeof(Program).GetMethod("Like",
            BindingFlags.Static | BindingFlags.NonPublic | BindingFlags.Public)
            ,lhs,rhs);
}

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