C# 创建动态表达式树

如何将这个Lambda转换为C中的表达式树#

到目前为止我已经试过了

private static Expression GetPropertyExpression(PropertyInfo prop, ParameterExpression paramExpr, ConstantExpression valueExpr) { 
    var memberAcc = Expression.MakeMemberAccess(paramExpr, prop); 
    Console.WriteLine(memberAcc); 
    var containsMember = typeof(string).GetMethod("Contains"); 
    var toLower= typeof(String).GetMethod("ToLower",new [] {typeof(string)}); 
    var ttt=Expression.Call(memberAcc,containsMember, valueExpr); 
    return Expression.Call(memberAcc,containsMember, valueExpr); 
}

---

你可以自己稍微简单一点。您可以使用

IndexOf

方法，而不是使用

Contains

方法，该方法不支持作为参数传递的

StringComparison

您可以这样实现它：

public static Expression<Func<T, bool>> ContainsValue<T>(string fieldName, string val) {
    var type = typeof(T);
    var member = Expression.Parameter(type, "param");
    var memberExpression = Expression.PropertyOrField( member, fieldName);
    var targetMethod = memberExpression.Type.GetMethod( "IndexOf", new Type[] { typeof(string), typeof(StringComparison) } );
    var methodCallExpression = Expression.Call( memberExpression, targetMethod, Expression.Constant(val), Expression.Constant( StringComparison.CurrentCultureIgnoreCase ) );

    return Expression.Lambda<Func<T, bool>>( 
        Expression.AndAlso(
            Expression.NotEqual(memberExpression, Expression.Constant(null)), 
            Expression.GreaterThanOrEqual( methodCallExpression, Expression.Constant(0) )
        ), 
        member
    );
}

您可以在上找到一个示例，但我也在下面添加了完整的代码

using System;
using System.Collections.Generic;
using System.Linq;
using System.Linq.Expressions;

public class Program
{
    public static Expression<Func<T, bool>> ContainsValue<T>(string fieldName, string val) {
        var type = typeof(T);
        var member = Expression.Parameter(type, "param");
        var memberExpression = Expression.PropertyOrField( member, fieldName);
        var targetMethod = memberExpression.Type.GetMethod( "IndexOf", new Type[] { typeof(string), typeof(StringComparison) } );
        var methodCallExpression = Expression.Call( memberExpression, targetMethod, Expression.Constant(val), Expression.Constant( StringComparison.CurrentCultureIgnoreCase ) );

        return Expression.Lambda<Func<T, bool>>( 
            Expression.AndAlso(
                Expression.NotEqual(memberExpression, Expression.Constant(null)), 
                Expression.GreaterThanOrEqual( methodCallExpression, Expression.Constant(0) )
            ), 
            member
        );
    }

    public static void Main()
    {
        var items = new List<Test>() {
            new Test() { Parameter = "Alpha" },
            new Test(),
            new Test() { Parameter = "Test" },
            new Test() { Parameter = "test" },
            new Test() { Parameter = "TEST" },
            new Test() { Parameter = "Contains test" }
        };
        var expr = ContainsValue<Test>("Parameter",  "test");
        // you can see the body here
        Console.WriteLine( expr.Body );
        // and the result
        var results = items.Where( expr.Compile() ).Select(t => t.Parameter).ToList();
        Console.WriteLine( "Results: {0}", string.Join( ",", results ));
        Console.WriteLine( "Total results: {0}", results.Count );
    }

    public class Test {
        public string Parameter { get;set; }
    }
}

---

到目前为止你尝试了什么？你被困在哪里了？我一直在尝试编写此代码来筛选任何列表。它可以在大写的私有静态表达式GetPropertyExpression（PropertyInfo-prop，ParameterExpression-ParameterExpr，ConstantExpression-valueExpr）{var memberAcc=Expression.MakeMemberAccess（ParameterExpr，prop）；//Console.WriteLine（memberAcc）；var containsMember=typeof（string）.GetMethod（“Contains”）；var toLower=typeof（string）.GetMethod（“toLower”，new[]{typeof（string）}）；var ttt=Expression.Call（memberAcc，containsMember，valuexpr）；return Expression.Call（memberAcc，containsMember，valuexpr）；}请将代码添加到问题中，而不是在注释中，至少在那里你可以用一种好的方式格式化它（使用你问题下的链接）谢谢Icepickle，我希望它能用于具有常见数据类型的类，如字符串、小数、双精度等。目前，它工作得很好，但在字符串字段上only@EmmanuelOgoma这当然是可能的，因为有许多不同的表达方式。然而，他们没有包含方法，例如，还有很多事情要做。根据您希望在以后如何组合表达式，这可能是相当多的工作；）这个lambda可以工作，但我不知道如何创建它的表达式。注意名为x=>x.FieldName的Tostring方法=null&&x.FieldName.Tostring（）.ToLower（）.Contains（param）@EmmanuelOgoma我认为您将其简化了一点，但当然，从您的问题来看，很难了解您的上下文。或者我认为它很复杂：）如果你愿意，我们可以考虑你的要求

((param.Parameter != null) AndAlso (param.Parameter.IndexOf("test", CurrentCultureIgnoreCase) >= 0))

using System;
using System.Collections.Generic;
using System.Linq;
using System.Linq.Expressions;

public class Program
{
    public static Expression<Func<T, bool>> ContainsValue<T>(string fieldName, string val) {
        var type = typeof(T);
        var member = Expression.Parameter(type, "param");
        var memberExpression = Expression.PropertyOrField( member, fieldName);
        var targetMethod = memberExpression.Type.GetMethod( "IndexOf", new Type[] { typeof(string), typeof(StringComparison) } );
        var methodCallExpression = Expression.Call( memberExpression, targetMethod, Expression.Constant(val), Expression.Constant( StringComparison.CurrentCultureIgnoreCase ) );

        return Expression.Lambda<Func<T, bool>>( 
            Expression.AndAlso(
                Expression.NotEqual(memberExpression, Expression.Constant(null)), 
                Expression.GreaterThanOrEqual( methodCallExpression, Expression.Constant(0) )
            ), 
            member
        );
    }

    public static void Main()
    {
        var items = new List<Test>() {
            new Test() { Parameter = "Alpha" },
            new Test(),
            new Test() { Parameter = "Test" },
            new Test() { Parameter = "test" },
            new Test() { Parameter = "TEST" },
            new Test() { Parameter = "Contains test" }
        };
        var expr = ContainsValue<Test>("Parameter",  "test");
        // you can see the body here
        Console.WriteLine( expr.Body );
        // and the result
        var results = items.Where( expr.Compile() ).Select(t => t.Parameter).ToList();
        Console.WriteLine( "Results: {0}", string.Join( ",", results ));
        Console.WriteLine( "Total results: {0}", results.Count );
    }

    public class Test {
        public string Parameter { get;set; }
    }
}

((param.Parameter != null) AndAlso (param.Parameter.IndexOf("test", CurrentCultureIgnoreCase) >= 0))
Results: Test,test,TEST,Contains test
Total results: 4