平等和分配操作员

Question

I have an assembly compiled in VB.NET that contains two operators:

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

However, when using the assembly in C# to perform equality checks if (qtr != null) I receive the error:

Cannot implicitly convert type 'object' to 'bool'

Usage in 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();}

What do I need to do to make the equality operator behave properly?

Answer 1

When I implement your code as-is and compare an instance to null I get a NullReferenceException in your equality operator. However if I add a null check it works fine:

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

I suspect something else is causing the error you're getting.

Most likely you're using the assignment operator ( = ) when you should be using the equality operator ( == ):

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

In addition, I would recommend overriding Equals and GetHashCode to be consistent with your equality operator.

Answer 2

You can't overload assignment operators, neither in VB or C#.

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