c# - 为什么这个使用的变量是灰色的？

Question

我有两个具有相同存在理由的本地变量 _lastShortName 和 _lastCompanyName ：

private string _lastShortName = String.Empty;
private string _lastCompanyName = String.Empty;

    ...which are used like this:

    private void AddFillRateByDCByLocationRow(FillRateByDCByLocation 
frbdcbl)
    {
        int rowToPopulate = 
_xlSheetFillRateByDCByLocation.UsedRange.Rows.Count + 1;

        // Don't want repeats of ShortName
        if (!_lastShortName.Equals(frbdcbl.ShortName))
        {
            var shortNameCell = 
(Excel.Range)_xlSheetFillRateByDCByLocation.Cells[rowToPopulate,
SHORTNAME_BYDCBYLOC_COL];
            shortNameCell.Value2 = frbdcbl.ShortName;
            shortNameCell.Font.Bold = true;
        }
        _lastShortName = frbdcbl.ShortName;

        // ... or CompanyName
        if (!_lastCompanyName.Equals(frbdcbl.CompanyName))
        {
            var companyNameCell = 
(Excel.Range)_xlSheetFillRateByDCByLocation.Cells[rowToPopulate, 
COMPANYNAME_BYDCBYLOC_COL];
            companyNameCell.Value2 = frbdcbl.CompanyName;
            companyNameCell.Font.Bold = true;
        }
        _lastCompanyName = frbdcbl.CompanyName;
        . . .

然而 _lastCompanyName 在它被声明的地方是灰色的。为什么会这样？Resharper > Inspect > Code Issues in Solution 并没有说它是一个未使用的变量，所以它必须是 Visual Studio 正在做灰色，但为什么呢？