c# - 否定表达式

Question

我有一个返回表达式的函数，我传入一个字段和一个值。我有返回 StartsWith 的函数，但我想返回 NOT StartsWith

private Expression<Func<T, bool>> GenerateFieldDoesNotStartsWithExpression<T>(string fieldName, string value) {
    var parameter = Expression.Parameter(typeof(T), "i");
    var fieldAccess = Expression.PropertyOrField(parameter, fieldName);
    MethodInfo method = typeof(string).GetMethod("StartsWith", new[] { typeof(string) });
    var passedValue = Expression.Constant(value, typeof(string));
    var body = Expression.Call(fieldAccess, method, passedValue);
    var expr = Expression.Lambda<Func<T, bool>>(body, parameter);
    return expr;
}

返回i.[fieldName].StartsWith(value)但我正在寻找 !i.[fieldName].StartsWith(value).

我尝试了一些方法，例如设置 parameter = !i 并将第二个参数设置为 i，但后来我得到“参数 '!i' 不是绑定(bind)”。

我尝试摆弄 Expression.Not，但似乎无法让它工作。

Answer 1

用Expression.Not包围你的Expression.Call

var body = Expression.Not(Expression.Call(fieldAccess, method, passedValue));

您的代码:

    private Expression<Func<T, bool>> GenerateFieldDoesNotStartsWithExpression<T>(string fieldName, string value)
    {
        var parameter = Expression.Parameter(typeof(T), "i");
        var fieldAccess = Expression.PropertyOrField(parameter, fieldName);
        MethodInfo method = typeof(string).GetMethod("StartsWith", new[] { typeof(string) });
        var passedValue = Expression.Constant(value, typeof(string));
        // Wrapped Expression.Call with Expression.Not
        var body = Expression.Not(Expression.Call(fieldAccess, method, passedValue));
        var expr = Expression.Lambda<Func<T, bool>>(body, parameter);
        return expr;
    }

测试代码:

[TestFixture]
public class ExpressNotTests
{
    [Test]
    public void GenerateFieldDoesNotStartsWithExpression_DoesNotStartWith_True()
    {
        var a = new TestClass() {TestString = "Not"};

        var exp = GenerateFieldDoesNotStartsWithExpression<TestClass>("TestString", "Test");

        var res = exp.Compile()(a);

        res.Should().BeTrue();
    }

    [Test]
    public void GenerateFieldDoesNotStartsWithExpression_DoesStartsWith_False()
    {
        var a = new TestClass() {TestString = "TestString"};

        var exp = GenerateFieldDoesNotStartsWithExpression<TestClass>("TestString", "Test");

        var res = exp.Compile()(a);

        res.Should().BeFalse();
    }


    private class TestClass
    {
        public string TestString { get; set; }
    }
}