LINQ成員表達越來越列名

Question

你好，

我使用LINQ和EF與C＃4.0。 我已經將基本的ELMAH表拖入EF（構建並保存很多次）。 所有工作正如人們所期望的那樣。

但試圖過於雄心勃勃，需要一些幫助 - 我想從傳遞中作爲一個變量表達式得到列名。

我想是這樣的：

通在：X => x.ErrorId

，並得到： 「ErrorID中」

public void GetColumnName(Expression<Func<T, object>> property) 
{ 
    // The parameter passed in x=>x.Message 
    // Message works fine (probably because its a simple string) using: 
    string columnName = (property.Body as MemberExpression).Member.Name; 

    // But if I attempt to use the Guid or the date field then it 
    // is passed in as x => Convert(x.TimeUtc) 
    // As a result the above code generates a NullReference exception 
    // i.e. {"Object reference not set to an instance of an object."} 

    // What is the correct code here to extract the column name generically? 
    // Ideally in a way that won't bite me again in the future. 

} 

謝謝您的幫助！ Dan。

Answer 1

如果您還需要分解簡單（或幾乎簡單）的表達式，你需要一些額外的跑腿來處理不同的情況。這裏是處理一些常見的情況下，某些啓動代碼：哪些生產同類產品的輸出

string GetColumnName<T,TResult>(Expression<Func<T,TResult>> property) 
{ 
    var member = GetMemberExpression(property.Body); 
    if (member == null) 
     throw new ArgumentException("Not reducible to a Member Access", 
            "property"); 

    return member.Member.Name; 
} 

MemberExpression GetMemberExpression(Expression body) 
{ 
    var candidates = new Queue<Expression>(); 
    candidates.Enqueue(body); 
    while (candidates.Count > 0) 
    { 
     var expr = candidates.Dequeue(); 
     if (expr is MemberExpression) 
     { 
      return ((MemberExpression)expr); 
     } 
     else if (expr is UnaryExpression) 
     { 
      candidates.Enqueue(((UnaryExpression)expr).Operand); 
     } 
     else if (expr is BinaryExpression) 
     { 
      var binary = expr as BinaryExpression; 
      candidates.Enqueue(binary.Left); 
      candidates.Enqueue(binary.Right); 
     } 
     else if (expr is MethodCallExpression) 
     { 
      var method = expr as MethodCallExpression; 
      foreach (var argument in method.Arguments) 
      { 
       candidates.Enqueue(argument); 
      } 
     } 
     else if (expr is LambdaExpression) 
     { 
      candidates.Enqueue(((LambdaExpression)expr).Body); 
     } 
    } 

    return null; 
} 

：

GetColumnName((x) => x.X): "X" 
GetColumnName((x) => x.X + 2): "X" 
GetColumnName((x) => 2 + x.X): "X" 
GetColumnName((x) => -x.X): "X" 
GetColumnName((x) => Math.Sqrt(x.Y)): "Y" 
GetColumnName((x) => Math.Sqrt(Math.Abs(x.Y))): "Y"