参数'***'未绑定到指定的LINQ to Entities查询表达式中 [英] The parameter '***' was not bound in the specified LINQ to Entities query expression

问题描述

我在我的项目中做了一个常见的查询。我使用表达式构建我的查询树，下面的代码列表：

  public IList< Book> GetBooksFields（string fieldName，string fieldValue）
 {
 ParameterExpression paramLeft = Expression.Parameter（typeof（string），m。+ fieldName）; 
 ParameterExpression paramRight = Expression.Parameter（typeof（string），\+ fieldValue +\）; 
 
 ParameterExpression binaryLeft = Expression.Parameter（typeof（Book），m）; 
 BinaryExpression binaryExpr = Expression.Equal（paramLeft，paramRight）; 
 
 var expr = Expression.Lambda&FunC< Book，bool>>（binaryExpr，binaryLeft）; 
 
 return bookRepository.GetMany（expr）.ToList（）; 
 
} 
  

但是当我调用我的 GetBooksFields 方法，它会抛出如下异常：

我调试了expr变量并得到了正确的表达式：{ m => （m.Name ==sdf）}，这是我想要的，但我不知道为什么我得到错误，thx。

解决方案

您不能通过将点投入变量名称来欺骗LINQ来将参数解释为成员表达式。 p>

您必须正确构造表达式树，如
（EDIT：根据您的评论将属性更改为属性）：

  public IList< Book> GetBooksFields（string propertyName，string propertyValue）
 {
 var parameter = Expression.Parameter（typeof（Book），book）; 
 
 var left = Expression.Property（parameter，propertyName）; 
 
 var convertedValue = Convert.ChangeType 
（
 propertyValue，
 typeof（Book）.GetProperty（propertyName）.PropertyType 
）; 
 
 var right = Expression.Constant（convertedValue）; 
 
 var binaryExpr = Expression.Equal（left，right）; 
 var expr = Expression.Lambda  
 return bookRepository.GetMany（expr）.ToList（）; 
} 
  

I am doing a common query in my project. I use Expression to build my query tree, the code list below:

 public IList<Book> GetBooksFields(string fieldName, string fieldValue)
    {
        ParameterExpression paramLeft = Expression.Parameter(typeof(string), "m." + fieldName);
        ParameterExpression paramRight = Expression.Parameter(typeof(string), "\"" + fieldValue + "\"");

        ParameterExpression binaryLeft = Expression.Parameter(typeof(Book),"m");
        BinaryExpression binaryExpr = Expression.Equal(paramLeft, paramRight);

        var expr = Expression.Lambda<Func<Book, bool>>(binaryExpr, binaryLeft);

        return bookRepository.GetMany(expr).ToList();

    }

But when I invoke my GetBooksFields method, it will throw me an exception as below:

I debugged the expr variable and got the correct expression: {m => (m.Name == "sdf")}, it was what I want, But I don't know why I got the error,thx.

解决方案

You can't "trick" LINQ into interpreting parameters as member-expressions by throwing in dots into variable names.

You'll have to construct the expression-tree correctly, as below (EDIT: changed field to property as per your comment):

public IList<Book> GetBooksFields(string propertyName, string propertyValue)
{
     var parameter = Expression.Parameter(typeof(Book), "book");

     var left = Expression.Property(parameter, propertyName);   

     var convertedValue = Convert.ChangeType
                          ( 
                              propertyValue, 
                              typeof(Book).GetProperty(propertyName).PropertyType
                          );

     var right = Expression.Constant(convertedValue);

     var binaryExpr = Expression.Equal(left, right);        
     var expr = Expression.Lambda<Func<Book, bool>>(binaryExpr, parameter);     

     return bookRepository.GetMany(expr).ToList();          
}

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