在if语句之前初始化对象

Question

在我的程序中,我正在阅读excel表并正在做一些linq选择,这工作正常.

问题:我尝试通过应用If语句进行预选.变量将在每种情况下分配(if/else),但编译器没有看到.编译器告诉我初始化var beforehands但是当我尝试这个时,我失败了因为我只习惯像sting,int或double之类的变量,我可以在之前轻松分配:

//This function takes the downloaded xlsx and makes selection of applicable items

var excel = new ExcelQueryFactory("list.xlsx");
//get all items with discount
if (onlyAcceptDiscountedItems == true)
{
    var discounts = from s in excel.Worksheet()
                    where s["Discount/Premium"].Cast<string>().StartsWith("-")
                    select s;
}
else
{
    var discounts = excel.Worksheet();
}
if (discounts.Count() != 0)
{
    //some application logic comes here
}

当我尝试这样做时:

var excel = new ExcelQueryFactory("list.xlsx");ter code here
var discounts = excel.Worksheet();
if (onlyAcceptDiscountedItems == true)
{
    discounts = from s in excel.Worksheet()
                where s["Discount/Premium"].Cast<string>().StartsWith("-")
                select s;
}
if (discounts.Count() != 0)
{
    //some application logic comes here
}

我在第二个codenippet的第5行收到以下错误:

错误CS0266无法将类型'System.Linq.IQueryable'隐式转换为'LinqToExcel.Query.ExcelQueryable'.存在显式转换(您是否错过了演员？)

Answer 1

我看到三个选项,尽管还有其他选项:

您可以显式声明所需的变量类型,在这种情况下,您可以在每种情况下单独初始化它:

IQueryable<LinqToExcel.Row> discounts;
if (onlyAcceptDiscountedItems == true)
{
    discounts = from s in excel.Worksheet()
                where s["Discount/Premium"].Cast<string>().StartsWith("-")
                select s;
}
else
{
    discounts = excel.Worksheet();
}

您可以使用条件运算符初始化变量:

var discounts = onlyAcceptDiscountedItems
    ? excel.Worksheet().Where(s => s["Discount/Premium"].Cast<string>().StartsWith("-"))
    : excel.Worksheet();

您可以excel.Worksheet在两个地方删除呼叫冗余:

IQueryable<LinqToExcel.Row> discounts = excel.Worksheet();
if (onlyAcceptDiscountedItems)
{
    discounts = discounts.Where(s => s["Discount/Premium"].Cast<string>().StartsWith("-"));
}

此外,您可能希望使用discounts.Any()而不是discounts.Count() > 0,因为您并不真正关心计数.