是否可以为字符串实现ExpressionTree.GreaterThan等，以便LINQ可以使用它

Question

今天早上我看到了一个问题（Query my model on a range of values）似乎由（https://stackoverflow.com/a/1447926/195550）回答，但整个情况让我对更广泛的解决方案感兴趣。

我希望能够使用Jon Skeet的答案来实现一个Between，它可以在非SQL生成环境中使用字符串键，但看起来字符串没有实现 GreaterThan ， LessThan ， GreaterThanOrEqual ， LessThanOrEqual 运算符阻碍了Linq能够构建必要的表达式树此

• 我意识到可以使用CompareTo方法进行查询来完成此任务，但我真的很喜欢query.Between(v=>v.StringKey, "abc", "hjk")表达式的优雅。

  < / LI>

• 我查看了System.Linq.Expression程序集，发现它正在寻找名为'op_GreaterThan'的方法，例如对于GreaterThan操作，但我不知道< / em>的

  1. 我是否可以为字符串实现此功能（知道我无法为字符串扩展实际的'＆gt;'运算符）
  2. 如何建立正确的方法签名。

• 我创建了以下示例和测试，显示了Between扩展方法对字符串键不起作用的地方。

如果可以为字符串键实现它，那将是非常优雅的。有人对如何做到这一点有任何建议或见解吗？

在Jon Skeet的运营商之间，添加了包含性标志

---

public static class BetweenExtension
{
    public static IQueryable<TSource> Between<TSource, TKey>(
        this IQueryable<TSource> source,
        Expression<Func<TSource, TKey>> keySelector,
        TKey low,
        TKey high,
        bool inclusive = true) where TKey : IComparable<TKey>
    {
        var key = Expression.Invoke(keySelector, keySelector.Parameters.ToArray());

        var lowerBound = (inclusive)
                  ? Expression.GreaterThanOrEqual(key, Expression.Constant(low))
                  : Expression.GreaterThan(key, Expression.Constant(low));

        var upperBound = (inclusive)
                  ? Expression.LessThanOrEqual(key, Expression.Constant(high))
                  : Expression.LessThan(key, Expression.Constant(high));

        var and = Expression.AndAlso(lowerBound, upperBound);
        var lambda = Expression.Lambda<Func<TSource, bool>>(
                        and, keySelector.Parameters);

        return source.Where(lambda);
    }
}

使用“int”键

进行上述工作测试

---

[TestFixture]
public class BetweenIntTests
{
    public class SampleEntityInt
    {
        public int SampleSearchKey { get; set; }
    }

    private IQueryable<SampleEntityInt> BuildSampleEntityInt(params int[] values)
    {
        return values.Select(
               value => 
               new SampleEntityInt() { SampleSearchKey = value }).AsQueryable();
    }

    [Test]
    public void BetweenIntInclusive()
    {
        var sampleData = BuildSampleEntityInt(1, 3, 10, 11, 12, 15);
        var query = sampleData.Between(s => s.SampleSearchKey, 3, 10);
        Assert.AreEqual(2, query.Count());
    }

    [Test]
    public void BetweenIntNotInclusive()
    {
        var sampleData = BuildSampleEntityInt(1, 3, 10, 11, 12, 15);
        var query = sampleData.Between(s => s.SampleSearchKey, 2, 11, false);
        Assert.AreEqual(2, query.Count());
    }
}

使用“字符串”键进行上述非工作测试

---

[TestFixture]
public class BetweenStringsTests
{

    public class SampleEntityString
    {
        public string SampleSearchKey { get; set; }
    }

    private IQueryable<SampleEntityString> BuildSampleEntityString(params int[] values)
    {
        return values.Select(
               value =>
               new SampleEntityString() {SampleSearchKey = value.ToString() }).AsQueryable();
    }

    [Test]
    public void BetweenStringInclusive()
    {
        var sampleData = BuildSampleEntityString(1, 3, 10, 11, 12, 15);
        var query = sampleData.Between(s => s.SampleSearchKey, "3", "10");
        Assert.AreEqual(2, query.Count());
    }

    [Test]
    public void BetweenStringNotInclusive()
    {
        var sampleData = BuildSampleEntityString(1, 3, 10, 11, 12, 15);
        var query = sampleData.Between(s => s.SampleSearchKey, "2", "11", false);
        Assert.AreEqual(2, query.Count());
    }
}

Answer 1

您必须在表达式树中调用string.CompareTo方法。然后，您可以测试其结果。要查看它的外观，请在调试器中查看以下值：

Expression<<Func<string, bool>> filter = str => str.CompareTo("abc") > 0;