在视图模型中有条件地更改显示名称

Question

我是 Razor 和 View Models 的新手，我只是想问一下是否可以在 [Display(Name = "")] 中显示不同的字符串

我尝试在显示和变量之间添加条件，但它显示错误

也试过了

public string Color {get;set;}
public String ColorDisplay
        {
            get
            {
                String name = "";
                if (ColorId == 25 || ColorId == 26)
                {
                    name = "Purple";
                }
                else
                {
                    name = "Green";
                }

                return name;
            }
        }

然后在我看来 @Html.LabelFor(m => m.ColorDisplay)

但似乎不起作用，因为它只是显示ColorDisplay

Answer 1

在这个问题中，您可能需要一个自定义属性来根据属性属性中提供的值更改文本。假设您想要这样的自定义属性用法：

[DisplayWhen("ColorId", 25, 26, "Purple", "Green")]
public String Color { get; set; }

并像这样使用 HTML 助手：

@Html.LabelFor(m => m.Color)

那么你应该做这些步骤：

1) 创建一个继承自Attribute 类的自定义属性。

public class DisplayWhenAttribute : Attribute
{
    private string _propertyName;
    private int _condition1;
    private int _condition2;
    private string _trueValue;
    private string _falseValue;

    public string PropertyName 
    {
       get
       {
           return _propertyName;
       }
    }

    public int Condition1
    {
       get
       {
           return _condition1;
       }
    }

    public int Condition2
    {
       get
       {
           return _condition2;
       }
    }

    public string TrueValue
    {
       get
       {
           return _trueValue;
       }
    }

    public string FalseValue
    {
       get
       {
           return _falseValue;
       }
    }

    public DisplayWhenAttribute(string propertyName, int condition1, int condition2, string trueValue, string falseValue)
    {
        _propertyName = propertyName;
        _condition1 = condition1;
        _condition2 = condition2;
        _trueValue = trueValue;
        _falseValue = falseValue;
    }
}

2) 创建自定义元数据提供程序类，检查自定义属性是否存在。

public class CustomModelMetadataProvider : DataAnnotationsModelMetadataProvider
{
    protected override ModelMetadata CreateMetadata(IEnumerable<Attribute> attributes, Type containerType, Func<object> modelAccessor, Type modelType, string propertyName)
    {
        var metadata = base.CreateMetadata(attributes, containerType, modelAccessor, modelType, propertyName);

        var additionalAttribute = attributes.OfType<DisplayWhenAttribute>().FirstOrDefault();

        if (additionalAttribute != null)
        {
            metadata.AdditionalValues.Add("DisplayWhenAttribute", additionalValues);
        }

        return metadata;
    }
}

3) 将CustomModelMetadataProvider 注册到 Global.asax 中的Application_Start() 方法中，如下所示：

protected void Application_Start()
{
    ModelMetadataProviders.Current = new CustomModelMetadataProvider();
}

4) 创建您自己的（或覆盖现有的）LabelFor 助手，以便它检查 DisplayWhenAttribute，如下例所示：

public static MvcHtmlString LabelFor<TModel, TProperty>(this HtmlHelper<TModel> helper, Expression<Func<TModel, TProperty>> expression, IDictionary<string, object> htmlAttributes)
{
    string result = string.Empty;

    var modelMetaData = expression.Compile().Invoke(helper.ViewData.Model);
    string fieldName = ExpressionHelper.GetExpressionText(expression);

    var containerType = typeof(TModel);
    var containerProperties = containerType.GetProperties();

    var propertyInfo = containerProperties.SingleOrDefault(x => x.Name == modelMetaData.PropertyName);
    var attribute = propertyInfo.GetCustomAttributes(false).SingleOrDefault(x => x is DisplayWhenAttribute) as DisplayWhenAttribute;

    var target = attribute.PropertyName; // target property name, e.g. ColorId
    var condition1 = attribute.Condition1; // first value to check
    var condition2 = attribute.Condition2; // second value to check

    var targetValue = (int)containerType.GetProperty(target).GetValue(helper.ViewData.Model);  

    // checking provided values from attribute
    if (targetValue == condition1 || targetValue == condition2)
    {
        result = attribute.TrueValue;
    }      
    else
    {
        result = attribute.FalseValue;
    }

    // create <label> tag with specified true/false value
    TagBuilder tag = new TagBuilder("label");
    tag.MergeAttributes(htmlAttributes);
    tag.Attributes.Add("for", helper.ViewContext.ViewData.TemplateInfo.GetFullHtmlFieldId(fieldName));
    tag.SetInnerText(result);

    return MvcHtmlString.Create(tag.ToString(TagRenderMode.Normal));
}

需要考虑的一些参考：

Is it possible to create conditional attribute as DisplayIf?

How to extend MVC3 Label and LabelFor HTML helpers?

MVC custom display attribute