通过自定义属性反映时出错

Question

我试图根据类的某些字段的名称和通过反射存在的自定义属性来获取类的某些字段的值。 我的自定义属性是：

 [AttributeUsage (AttributeTargets.All, AllowMultiple=true)]
public sealed class ColumnAttribute : Attribute
{
    internal string name = "";
    internal string length = "";
    internal string precision = "";

    public ColumnAttribute() { }
    public ColumnAttribute(String name) { this.name = name; }
    public ColumnAttribute(String name, String length) { }
    public ColumnAttribute(String name, String length, String precision) { }

    public String Name { get { return name; } set { name = value; } }
    public String Length { get { return length; } set { length = value; } }
    public String Precision { get { return precision; } set { precision = value; } }
}

使用此属性的示例类是：

class SampleEntity
{
    //private int number;
    public string name;
    //float marks;

    public virtual int Number { get; set; }
    public SampleEntity() { }
    public SampleEntity(int number)
    {
        this.Number = number;
    }
    public void conversation(string request, string response) { }

    public void ordinary() {
        Console.Write("This isn't ordinary...");
    }
    [ColumnAttribute (Name = "XWBCCD")]
    public String XWBCCD { get; set; }

    [ColumnAttribute (Name = "XWBNCD")]
    public String XWBNCD { get; set; }

我还有一个具有不同字段名称的不同类：

 class SampleRepository
{

    [ColumnAttribute(Name = "XWBCCD")]
    public String SomeOtherFieldName { get; set; }

    [ColumnAttribute(Name = "XWBNCD")]
    public String XWBNCD { get; set; }

    [ColumnAttribute(Name = "XWBWCD")]
    public String XWBWCD { get; set; }
}

通过反射，我尝试通过匹配属性“名称”参数而不是字段名称来复制值。 问题是，在反射期间，这种比较不会通过传递字段的 getCustomAttributes() 方法进行。 我解决这个问题的方法（到目前为止失败）是： 首先，我传递 2 个对象，objSrc（第一类，已填充）和 objDesc（第二类，空）

 FieldInfo[] srcFields = objSrc.GetType().GetFields(BindingFlags.NonPublic | BindingFlags.Public | BindingFlags.Instance | BindingFlags.GetProperty | BindingFlags.IgnoreCase |BindingFlags.FlattenHierarchy);
        FieldInfo[] destFields = objDest.GetType().GetFields(BindingFlags.NonPublic | BindingFlags.Public | BindingFlags.Instance | BindingFlags.GetProperty);

然后我尝试迭代地对所有字段执行反射

foreach (FieldInfo srcFld in srcFields)
        {
            foreach (FieldInfo destFld in destFields)
            {
                if (((MemberInfo)srcFld).Name.Equals(((MemberInfo)destFld).Name)){
                    destFld.SetValue(objDest, srcFld.GetValue(objSrc));
                    break;
                }
                object[] srcAttr = srcFld.GetCustomAttributes(true);
                object[] destAttr = destFld.GetCustomAttributes(true);

                if (Utils.Length(srcAttr) == 1 && Utils.Length(destAttr) == 1){

                    if ((srcAttr[0]).Equals(destAttr[0]) && srcFld.FieldType.Equals(destFld.FieldType))
                        destFld.SetValue(objDest, srcFld.GetValue(objSrc));
                    else
                        break;
                }

            }
        }

GetCustomAttributes() 方法出现问题，因为它返回 null 。

Answer 1

我为 vb.net 代码而不是 c# 表示歉意，但我碰巧有一个 linqpad 文件，我在其中做完全相同的事情，在这里：（

根本不是真正的生产就绪代码，但它让我明白了这个想法希望。）

Sub Main

    dim data as DataRow = GetSomeData.GetData(2311385).rows(0)

    dim dm as IDataMapper = new DataMapper()

    dim login as LoginData = dm.MapDataTo(of LoginData)(data)

    data.Dump()
    login.dump()

End Sub

public interface IDataMapper
    function MapDataTo(of T as {IDataMappable,new})(data as DataRow) as T
end interface

public interface IDataMappable
end interface

public class DataMapper
    implements IDataMapper
    Public Function MapDataTo(Of T As {New, IDataMappable})(ByVal data As System.Data.DataRow) As T Implements IDataMapper.MapDataTo
        dim mapToObj as new T
        dim properties() as PropertyInfo = gettype(T).GetProperties()
        for each propInfo as PropertyInfo in properties
            Dim attributes() As Attribute = propInfo.GetCustomAttributes(GetType(DataMappingAttribute), True)
            if attributes.length > 0 then
                Dim dataAttribute As DataMappingAttribute = CType(attributes(0), DataMappingAttribute)
                Dim column As String = dataAttribute.ColumnName
                Dim dataType As Type = dataAttribute.DataType
                propInfo.SetValue(mapToObj, Convert.ChangeType(data(column), dataType), Nothing)
            end if
        next
        return mapToObj
    end function
end class

<AttributeUsage(AttributeTargets.Property)> _
public class DataMappingAttribute
    inherits Attribute
    private _ColumnName as string
    Public readonly Property ColumnName() As String
        Get
            return _ColumnName
        End Get
    End Property
    private _DataType as Type
    Public readonly Property DataType() As Type
        Get
            return _DataType
        End Get
    End Property
    public sub new(ColumnName as string, DataType as Type)
        me._columnName = ColumnName
        me._DataType = DataType
    end sub 
end class


public class LoginData
    implements IDataMappable

    private _LoginID as integer
    <DataMappingAttribute("loginID",Gettype(integer))> _
    public property LoginID as integer
        get
            return _LoginID
        end get
        set(value as integer)
            _LoginID = value
        end set
    end property

    private _LoginName as string
    <DataMappingAttribute("loginName",Gettype(String))> _
    public property LoginName as string
        get
            return _LoginName
        end get
        set(value as string)
            _LoginName = value
        end set
    end property

    private _FirstName as string
    <DataMappingAttribute("firstName",Gettype(String))> _
    public property FirstName as string
        get
            return _FirstName
        end get
        set(value as string)
            _FirstName = value
        end set
    end property

    private _LastName as string
    <DataMappingAttribute("lastName",Gettype(String))> _
    public property LastName as string
        get
            return _LastName
        end get
        set(value as string)
            _LastName = value
        end set
    end property

    private _EmailAddress as string
    <DataMappingAttribute("emailAddress",Gettype(String))> _
    public property EmailAddress as string
        get
            return _EmailAddress
        end get
        set(value as string)
            _EmailAddress = value
        end set
    end property

end class

public class GetSomeData
    Public shared Function GetData(ByVal ID As Integer) As DataTable
            'return a datatable
    End Function
end class