Question

我有一个用VB.NET编译的程序集，它包含两个运算符：

Public Shared Operator =(quarterA As CalendarQuarter, quarterB As CalendarQuarter) As Boolean
    Return quarterA.StartDate = quarterB.StartDate AndAlso
        quarterA.EndDate = quarterB.EndDate AndAlso
        quarterA.Quarter = quarterB.Quarter
End Operator

Public Shared Operator <>(quarterA As CalendarQuarter, quarterB As CalendarQuarter) As Boolean
    Return Not (quarterA = quarterB)
End Operator

但是，在C＃中使用程序集执行相等性检查时if (qtr != null)我收到错误：

Cannot implicitly convert type 'object' to 'bool'

用于C＃MVC4，Razor：

@{Html.BeginForm();}
    <div class="ui-form ui-form-horizontal form-width-narrow">
        <div class="title">
            Choose a Quarter</div>
        <div class="group">
            <label><strong>Control</strong></label>
            <div class="field">
                @Html.DropDownListFor(x => x.Quarter, new SelectList(Model.AvailableQuarters))
                <input value="Select" class="ui-button" type="submit" />
            </div>
        </div>
        @if (Model.Quarter != null) {
            // Error in the above statement 
        }           
    </div>
@{Html.EndForm();}

如何使等式运算符正常运行？

Answer 1

当我按原样实现你的代码并将一个实例比较为null时，我在你的相等运算符中得到一个NullReferenceException。但是，如果我添加一个空检查它可以正常工作：

Public Shared Operator =(quarterA As CalendarQuarter, quarterB As CalendarQuarter) As Boolean

    If quarterA Is Nothing OrElse quarterB Is Nothing Then Return False

    Return quarterA.StartDate = quarterB.StartDate AndAlso
        quarterA.EndDate = quarterB.EndDate AndAlso
        quarterA.Quarter = quarterB.Quarter
End Operator

我怀疑还有其他因素导致你得到的错误。

当你应该使用等于运算符（=）时，很可能你正在使用赋值运算符（==）：

if (qtr = null)   // wrong  - assigning null to qtr
if (qtr == null)  // correct

此外，我建议overriding Equals and GetHashCode与您的相等运算符保持一致。

Answer 2

您不能在VB或C＃中重载赋值运算符。

http://msdn.microsoft.com/en-us/library/8edha89s.aspx

平等和分配操作员

2 个答案: