创建表达式< Func< T，对象>>从给定的类型 [英] Create Expression<Func<T, object>> from a given Type

问题描述

我正在尝试传递这些表达式的方法具有以下签名：

  public virtual CsvPropertyMap< TClass，TProperty>地图< TProperty>（表达< Func< TClass，TProperty>>表达式）
 {
 // 
} 
  

所以你通常会把它称为，你想要映射的任何给定类型，像这样（对于具有名为stringProperty的属性的类型）：

  mapper.Map（x => x.StringProperty）;传递一个内部转换成一个表达式< Func< T中的lambda的
 



，对象>>

我已尝试使用表达式在代码中创建此表达式。在编译时，它一切正常（因为它返回一个 Expression< Func< TModel，object>> ），但在运行时我收到异常'不是成员访问。以下是代表我要映射的属性的PropertyInfo对象的代码：

  private Expression&FunC< TModel，object> > CreateGetterExpression（PropertyInfo propertyInfo）
 {
 var getter = propertyInfo.GetGetMethod（）; 
 
表达式< Func< TModel，object>> expression = m => getter.Invoke（m，new object [] {}）; 
返回表达式; 
} 
  

基本上，如何在代码中正确构建Expression？

解决方案

只需尝试看起来像这样：

  public static Expression< Func< T，P>> GetGetter< T，P>（string propName）
 {
 var parameter = Expression.Parameter（typeof（T））; 
 var property = Expression.Property（parameter，propName）; 
 return Expression.Lambda< Func< T，P>>（property，parameter）; 
} 
 
 public static Expression< Func< T，P>> GetGetter< T，P>（PropertyInfo propInfo）
 {
 var parameter = Expression.Parameter（typeof（T））; 
 var property = Expression.Property（parameter，propInfo）; 
 return Expression.Lambda< Func< T，P>>（property，parameter）; 
} 
  

这是用法的例子：

 私人类TestCalss 
 {
 public int Id {get;组; 
} 
 
 private static void Main（string [] args）
 {
 var getter = GetGetter&TestCalss，int>（typeof（TestCalss）.GetProperty ID））编译（）; 
 Console.WriteLine（getter（new TestCalss {Id = 16}））; 
} 
  

I am looking to use CsvHelper dynamically by building up Expressions in code which represent property member access for a given type.

The method I am trying to pass these expressions to has the following signature:

    public virtual CsvPropertyMap<TClass, TProperty> Map<TProperty>( Expression<Func<TClass, TProperty>> expression )
    {
        //
    }

So you would normally call it, for any given type you want to map, like this (for a type with a property called 'stringProperty'):

mapper.Map(x => x.StringProperty);

Passing in a lambda which is converted internally into an Expression<Func<T, object>>

I have tried to create this expression in code, using Expressions. At compile time it all works fine (in that it returns an Expression<Func<TModel, object>>), but at runtime I get an exception 'not a member access'. Here is the code which takes a PropertyInfo object representing the property I want to map:

    private Expression<Func<TModel, object>> CreateGetterExpression( PropertyInfo propertyInfo )
    {
        var getter = propertyInfo.GetGetMethod();

        Expression<Func<TModel, object>> expression = m => getter.Invoke( m, new object[] { } );
        return expression;
    }

Basically, how do I build that Expression up properly in code?

解决方案

Just try something looks like this:

    public static Expression<Func<T, P>> GetGetter<T, P>(string propName)
    {
        var parameter = Expression.Parameter(typeof(T));
        var property = Expression.Property(parameter, propName);
        return Expression.Lambda<Func<T, P>>(property, parameter);
    }

    public static Expression<Func<T, P>> GetGetter<T, P>(PropertyInfo propInfo)
    {
        var parameter = Expression.Parameter(typeof(T));
        var property = Expression.Property(parameter, propInfo);
        return Expression.Lambda<Func<T, P>>(property, parameter);
    }

It's example of usages:

    private class TestCalss
    {
        public int Id { get; set; }
    }

    private static void Main(string[] args)
    {
        var getter = GetGetter<TestCalss, int>(typeof(TestCalss).GetProperty("Id")).Compile();
        Console.WriteLine(getter(new TestCalss { Id = 16 }));
    }

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