c# - 从 ConstantExpression 获取值

Question

我正在寻找一个值

var guid = Guid.Parse("SOMEGUID-GUID-GUID-GUID-SOMEGUIDGUID");
Expression<Func<Someobject, bool>> selector = x => x.SomeId == guid;

出于日志记录的目的，我需要能够找出该 guid。

我尝试了以下代码，我觉得它有点接近我正在寻找的东西，但又不完全是。

BinaryExpression binaryExpression = (BinaryExpression)selector.Body;
MemberExpression memberExpression = (MemberExpression)((UnaryExpression)binaryExpression.Right).Operand;
ConstantExpression constantExpression = (ConstantExpression)memberExpression.Expression;

现在，ConstantExpression 公开了一个成员“Value”，它确实包含我正在寻找的内容，但我有点困惑如何实际提取它。

没有:

var val = (Guid)constantExpression.Value; 

不起作用:)

已解决

最终结果如下:

BinaryExpression binaryExpression = (BinaryExpression)selector.Body;
MemberExpression memberExpression = (MemberExpression)((UnaryExpression)binaryExpression.Right).Operand;
var myGuid = Expression.Lambda(memberExpression).Compile().DynamicInvoke();

跟进

我使用以下代码进行了一些粗鲁的速度测试:

    static void Main(string[] args)
    {
        var id = Guid.Parse("bleh");

        Expression<Func<Thingemebob, bool>> selector = x => x.Id == id;

        var tickList = new List<long>();

        for (int i = 0; i < 100000; i++)
        {
            var sw = Stopwatch.StartNew();
            GetValueWithExpressionsAndReflection(selector);
            sw.Stop();
            tickList.Add(sw.ElapsedTicks);
        }

                    Trace.WriteLine("GetValueWithExpressionsAndReflection: Average over 100000, first call included: " + tickList.Average());
        Trace.WriteLine("GetValueWithExpressionsAndReflection: First call: " + tickList[0]);
        Trace.WriteLine("GetValueWithExpressionsAndReflection: Average over 100000, first call excluded: " + tickList.Skip(1).Average());

        tickList = new List<long>();

        for (int i = 0; i < 100000; i++)
        {
            var sw = Stopwatch.StartNew();
            GetValueWithCompiledExpression(selector);
            sw.Stop();
            tickList.Add(sw.ElapsedTicks);
        }

                    Trace.WriteLine("GetValueWithCompiledExpression: Average over 100000, first call included: " + tickList.Average());
        Trace.WriteLine("GetValueWithCompiledExpression: First call: " + tickList[0]);
        Trace.WriteLine("GetValueWithCompiledExpression: Average over 100000, first call excluded: " + tickList.Skip(1).Average());

        Debugger.Break();
    }

    private static void GetValueWithCompiledExpression(Expression<Func<Note, bool>> selector)
    {
        BinaryExpression binaryExpression = (BinaryExpression)selector.Body;
        MemberExpression memberExpression = (MemberExpression)((UnaryExpression)binaryExpression.Right).Operand;
        var o = Expression.Lambda(memberExpression).Compile().DynamicInvoke();
    }

    private static void GetValueWithExpressionsAndReflection(Expression<Func<Note, bool>> selector)
    {
        BinaryExpression binaryExpression = (BinaryExpression)selector.Body;
        MemberExpression memberExpression = (MemberExpression)((UnaryExpression)binaryExpression.Right).Operand;
        ConstantExpression constantExpression = (ConstantExpression)memberExpression.Expression;

        FieldInfo member = (FieldInfo)memberExpression.Member;
        var instance = constantExpression.Value;
        var guid = member.GetValue(instance);
    }

事实证明编译版本要慢得多。我们正在寻找一个巨大的差异。 (时间以滴答为单位):

GetValueWithExpressionsAndReflection: Average over 100000, first call included: 0,93122

GetValueWithExpressionsAndReflection: First call: 851

GetValueWithExpressionsAndReflection: Average over 100000, first call excluded: 0,922719227192272

对比:

GetValueWithCompiledExpression: Average over 100000, first call included: 499,53669

GetValueWithCompiledExpression: First call: 16818

GetValueWithCompiledExpression: Average over 100000, first call excluded: 499,373503735037

是否进行基本测试:毫无疑问，我将使用反射版本。我的结果似乎与以下内容一致: http://www.minddriven.de/index.php/technology/dot-net/c-sharp/efficient-expression-values

Answer 1

您的 const 表达式的类型为 EFAndExpressions.Program+<>c__DisplayClass0 .这意味着表达式具有以下结构:

var compilerGeneratedClass = new compilerGeneratedClass() {
   guid = Guid.Parse("SOMEGUID-GUID-GUID-GUID-SOMEGUIDGUID"); };
Expression<Func<Someobject, bool>> selector = x => x.SomeId == compilerGeneratedClass.guid;

编译器会为你做这件事。使用反编译器查看详细信息。

现在你知道表达式树的样子了，你可以分解它了。您需要使用反射来获取 compilerGeneratedClass.guid 的运行时值字段或属性。

这个值不直接是表达式树的一部分。