表达式树:可以使用动态表达式方法构建

本文关键字：表达式 方法 构建 动态 可以使 | 更新日期: 2023-09-27 17:58:49

我是表达式树的新手，一直在尝试构建动态表达式，以便在Linq-to-entities查询中的.Where()方法中使用。当我显式调用像Expression.Equal(exp, exp)和Expression.GreaterThan(exp, exp)这样的Expression方法时，我可以让它全部工作。我想做的是不必对Expression.方法进行硬编码，这样我就可以传递一个带有方法名称的字符串，并用它动态构建。我下面有一个例子。这可能吗？我不知道该怎么办。

 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();
 }

表达式树:可以使用动态表达式方法构建

原来我只需要调用这个方法！我会提出这个解决方案，以防其他人从中受益。

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));
  }
}
相关文章：