Question

我是表达树的新手，并且一直在尝试构建动态表达式，以便在Linq中的.Where()方法中使用实体查询。当我明确调用像Expression.Equal(exp, exp)和Expression.GreaterThan(exp, exp)这样的表达式方法时，我可以全部工作。我想要做的是不必对Expression.Method进行硬编码，以便我可以传递一个带有方法名称的字符串并使用它动态构建。我有一个例子如下。这可能吗？我不知道如何继续。

 static void Main(string[] args)
 {

   int i = 1;
   int? iNullable = 1;
   object o = 1;
   int j = 2;

   ParameterExpression pe1 = Expression.Parameter(typeof(int));
   ParameterExpression pe2 = Expression.Parameter(typeof(int));

   //explicitly creating expression by calling Expression.Method works fine
   Expression call = Expression.Equal(pe1, pe2);
   var f = Expression.Lambda<Func<int, int, bool>>(call, pe1, pe2).Compile();

   Console.WriteLine(f(i, i));
   Console.WriteLine(f((int)iNullable, i));
   Console.WriteLine(f(i, (int)o));
   Console.WriteLine(f(i, j));

   //I want to use a dynamic Expression method instead of Expression.Equal
   //so that it could be substituted with Expression.GreaterThan etc.
   String expressionMethod = "Equal";

   //get the method
   MethodInfo method = typeof(Expression)
     .GetMethod(expressionMethod, 
      new[] { typeof(Expression), typeof(Expression) });

   //I'm lost ....

    Console.ReadKey();

 }

Answer 1

原来我只需要调用该方法！我会提出这个解决方案，以防其他任何人从中受益。

class Program
{
  static void Main(string[] args)
  {

    int i = 1;
    int? iNullable = 1;
    object o = 1;
    int j = 2;

    ParameterExpression pe1 = Expression.Parameter(typeof(int));
    ParameterExpression pe2 = Expression.Parameter(typeof(int));

    //explicitly creating expression by calling Expression.Method works fine
    Expression call = Expression.Equal(pe1, pe2);
    var f = Expression.Lambda<Func<int, int, bool>>(call, pe1, pe2).Compile();

    Console.WriteLine(f(i, i));
    Console.WriteLine(f((int)iNullable, i));
    Console.WriteLine(f(i, (int)o));
    Console.WriteLine(f(i, j));

    //I want to use a dynamic Expression method instead of Expression.Equal
    //so that it could be substituted with Expression.GreaterThan etc.
    List<String> expressionMethod = new List<string>(){"Equal", "GreaterThan"};

    foreach (String s in expressionMethod) DynamicExpression(s, j, i); 


    Console.ReadKey();
  }

  static void DynamicExpression(String methodName, int n1, int n2)
  {
    ParameterExpression pe1 = Expression.Parameter(typeof(int));
    ParameterExpression pe2 = Expression.Parameter(typeof(int));

    //get the method
    MethodInfo method = typeof(Expression)
        .GetMethod(methodName,
         new[] { typeof(Expression), typeof(Expression) });

    //Invoke
    Expression dynamicExp = (Expression)method.Invoke(null, new object[] { pe1, pe2 });
    var f = Expression.Lambda<Func<int, int, bool>>(dynamicExp, pe1, pe2).Compile();

    Console.WriteLine("Result for " 
         + n1.ToString() + " " 
         + methodName + " " + n2.ToString() + ": " + f(n1, n2));
  }
}

表达式树：可以用动态表达方法构建吗？

1 个答案: