在Linq的表达式树到实体

Question

我试图做一个扩展方法，它接受一个表达式指向一个实体的属性，以及一个常量字符串来比较它使用String.Contains，它首先检查字符串是空或空，并且仅在字符串具有值时应用过滤器。这是我在表达树中的第一次尝试，所以我不确定发生了什么，现在我有一个例外，我不知道如何治疗...

我在这个目前为止：

<System.Runtime.CompilerServices.Extension()> 
    Public Function CheckAndFilter(Of T)(source As System.Linq.IQueryable(Of T), expressionField As System.Linq.Expressions.Expression(Of System.Func(Of T, String)), compareTo As String) As IQueryable(Of T) 


     If String.IsNullOrEmpty(compareTo) Then 
      Return source 
     Else 
      Dim memberExp As Expressions.MemberExpression = DirectCast(expressionField.Body, Expressions.MemberExpression) 

      Dim param = Expressions.Expression.Parameter(GetType(T)) 
      Dim method As Reflection.MethodInfo = GetType(String).GetMethod("Contains") 

      Dim compareToExpression = Expressions.Expression.Constant(compareTo) 
      Dim finalExpr = Expressions.Expression.Call(memberExp, method, compareToExpression) 

      Dim lambda = Expressions.Expression.Lambda(Of Func(Of T, Boolean))(finalExpr, param) 
      Return source.Where(lambda) 
     End If 
    End Function 

我打电话是这样，对一个DbContext在那里我有一个Customer实体与FirstName字符串属性：

Dim results = repository.Customers.CheckAndFilter(Function(c) c.FirstName, searchText) 

，异常为：

{"The parameter 'c' was not bound in the specified LINQ to Entities query expression."} 

有什么想法？

Answer 1

哈哈！

这是因为我已经宣布了新的参数，而不是重新使用一个通过表达传递...

<System.Runtime.CompilerServices.Extension()> 
Public Function CheckAndFilter(Of T)(source As System.Linq.IQueryable(Of T), expressionField As System.Linq.Expressions.Expression(Of System.Func(Of T, String)), compareTo As String) As IQueryable(Of T) 


    If String.IsNullOrEmpty(compareTo) Then 
     Return source 
    Else 
     Dim memberExp As Expressions.MemberExpression = DirectCast(expressionField.Body, Expressions.MemberExpression) 

     Dim method As Reflection.MethodInfo = GetType(String).GetMethod("Contains") 

     Dim compareToExpression = Expressions.Expression.Constant(compareTo) 
     Dim finalExpr = Expressions.Expression.Call(memberExp, method, compareToExpression) 

     Dim lambda = Expressions.Expression.Lambda(Of Func(Of T, Boolean))(finalExpr, expressionField.Parameters) 
     Return source.Where(lambda) 
    End If 
End Function