Question

我需要将值从一种类型映射到另一种类型。两个对象都已经实例化并具有值。一个是MVC ViewModel对象，另一个是NHibernate实体。我想将视图模型映射到实体，但我必须仅在它们不同时才分配值。我还想记录更改。我想整齐地包装成映射方法/函数。我不认为使用AutoMapper是可行的。是吗？我也想出了自定义映射函数，但还有更好的方法吗？

Answer 1

我认为我更喜欢它作为目标对象的扩展名：）

/// <summary>
/// Maps a property value from one object type to another.
/// Assignment only if values are different.
/// Change is recorded to stringbuilder object with changeName name
/// </summary>
/// <typeparam name="TDestination">Type of destination object</typeparam>
/// <typeparam name="TSourceValue">Type of source object</typeparam>
/// <param name="changeName">what to call the change for logging</param>
/// <param name="dObj">the desitnation object instance on which to call the extension method</param>
/// <param name="destProp">destination (old value) property</param>
/// <param name="sVal">the source (new) value</param>
/// <param name="sbChanges">where to log the changes</param>
/// <returns>1 if values were different, 0 if values were the the same</returns>
/// <example>
///     var sbChanges = new StringBuilder();
///     var c = 0;
///     c += DestObj.MapChange("My Property", x => x.DestProp, SourceObj.SourceProp, sbChanges);
/// </example>
public static short MapChange<TDestination, TSourceValue>(this TDestination dObj, string changeName, Expression<Func<TDestination, object>> destProp,
                                                      TSourceValue sVal, StringBuilder sbChanges)
{
    var dExpr = destProp.Body as MemberExpression ?? ((UnaryExpression)destProp.Body).Operand as MemberExpression;

    if (dExpr == null)
        throw new Exception("Destination expression must be a field or property");

    var dProp = typeof(TDestination).GetProperty(dExpr.Member.Name);

    var dVal = dProp.GetValue(dObj, null);

    //Equals(null, null) is false so also compare by reference
    if (!ReferenceEquals(dVal, sVal) && !Equals(dVal, sVal))
    {
        dProp.SetValue(dObj, sVal, null);

        if (sVal == null) // http://stackoverflow.com/questions/5340817/what-should-i-do-about-possible-compare-of-value-type-with-null
            sbChanges.AppendFormat("Removed '{0}'\r", changeName);
        else if (dVal == null || sVal is bool)
            sbChanges.AppendFormat("Set '{0}' to '{1}'\r", changeName, sVal);
        else //other types
            sbChanges.AppendFormat("Set '{0}' from '{1}' to '{2}'\r", changeName, dVal, sVal);

        return 1;
    }

    return 0;
}

自定义属性映射 - 仅在更改时的现有对象

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