为什么我得到，“不能将类型'void'隐式转换为”对象“用这个代码（EPPlus）？ [英] Why am I getting, "Cannot implicitly convert type 'void' to 'object'" with this code (EPPlus)?

问题描述

我使用这个代码使用EPPlus在电子表格中填充单元格：

  using（var footerMonth1Cell = prodUsageWorksheet.Cells [columnFooterRow，MONTH1_COL]）
 {
 footerMonth1Cell.Value = monthsTruncatedYears [0]; 
 footerMonth1Cell.Style.Font.Size = DATA_FONT_SIZE; 
 footerMonth1Cell.Style.Horizo​​ntalAlignment = ExcelHorizo​​ntalAlignment.Right; 
} 
  

我想，工作正常，但是一个用户抱怨说它正在生产绿色三角形在细胞中。 monthTruncatedYears是一个通用的字符串列表。

所以我添加了这个帮助函数：

  public static void SetValueIntOrStr（此ExcelRangeBase范围，对象值）
 {
 string strVal = value.ToString（）; 
 if（！String.IsNullOrEmpty（strVal））
 {
 double dVal; 
 int iVal; 
 
 if（double.TryParse（strVal，out dVal））
 range.Value = dVal; 
 else if（Int32.TryParse（strVal，out iVal））
 range.Value = iVal; 
 else 
 range.Value = strVal; 
} 
 else 
 range.Value = null; 
} 
  

...我从这里，然后尝试重构原始代码来调用帮助函数如下所示：

  using（var footerMonth1Cell = prodUsageWorksheet.Cells [columnFooterRow，MONTH1_COL]）
 {
 footerMonth1Cell.Value = SetValueIntOrStr（footerMonth1Cell，monthsTruncatedYears [0]）; 
 footerMonth1Cell.Style.Font.Size = DATA_FONT_SIZE; 
 footerMonth1Cell.Style.Horizo​​ntalAlignment = ExcelHorizo​​ntalAlignment.Right; 
} 
  

但是，它不会编译，告诉我，不能由于第二个arg传递给SetValueIntOrStr（），即value，是类型对象，因此，将void转换为object

<我认为这是问题。那么为什么编译器看起来将MonthTruncatedYears [0]视为无效？在遗留代码中，我把它分配给单元格的值，当时它不是无效的，所以...？！？

我试过第二个对象就是这样：

  footerMonth1Cell.Value = ReportRunnerConstsAndUtils.SetValueIntOrStr（footerMonth1Cell，object（monthsTuncatedYears [0]））; 
  

...但是也不会用无效的表达式术语对象

...这种类似的尝试：

  footerMonth1Cell.Value = ReportRunnerConstsAndUtils.SetValueDoubleOrIntOrStr（footerMonth1Cell，monthsTruncatedYears [0] as object）; 
  

...引用，无法将类型'void'隐式转换为object

请注意，助手方法实际上在借用的代码中被错误地命名;而不是SetValueIntOrStr，它应该是SetValueDoubleOrIntOrStr

解决方案

问题是线路

  footerMonth1Cell.Value = SetValueIntOrStr（footerMonth1Cell，monthsTruncatedYears [0]）; 
  

SetValueIntOrStr 不返回任何值），因此不能用于将值分配给 footerMonth1Cell.Value 。

这将是有效的代码，因为Value属性已经在函数内部更改：

  SetValueIntOrStr（footerMonth1Cell，monthsTruncatedYears [0]）; 
  

I was using this code to populate a cell in a spreadsheet using EPPlus:

using (var footerMonth1Cell = prodUsageWorksheet.Cells[columnFooterRow, MONTH1_COL])
{
    footerMonth1Cell.Value = monthsTruncatedYears[0];
    footerMonth1Cell.Style.Font.Size = DATA_FONT_SIZE;
    footerMonth1Cell.Style.HorizontalAlignment = ExcelHorizontalAlignment.Right;
}

It was working fine, I thought, but a user complained that it was producing "green triangles" in the cells. "monthsTruncatedYears" is a generic list of string.

So I added this helper function:

public static void SetValueIntOrStr(this ExcelRangeBase range, object value)
{
    string strVal = value.ToString();
    if (!String.IsNullOrEmpty(strVal))
    {
        double dVal;
        int iVal;

        if (double.TryParse(strVal, out dVal))
            range.Value = dVal;
        else if (Int32.TryParse(strVal, out iVal))
            range.Value = iVal;
        else
            range.Value = strVal;
    }
    else
        range.Value = null;
}

...which I got from here, and then tried to refactor the original code to call that helper function like so:

using (var footerMonth1Cell = prodUsageWorksheet.Cells[columnFooterRow, MONTH1_COL])
{
    footerMonth1Cell.Value = SetValueIntOrStr(footerMonth1Cell, monthsTruncatedYears[0]);
    footerMonth1Cell.Style.Font.Size = DATA_FONT_SIZE;
    footerMonth1Cell.Style.HorizontalAlignment = ExcelHorizontalAlignment.Right;
}

However, it won't compile, telling me, "Cannot implicitly convert type 'void' to 'object'"

Since the second arg passed to SetValueIntOrStr(), namely "value", is of type object, I assume that is the problem. So why does the compiler apparently view monthsTruncatedYears[0] as being void? In the legacy code I assigned it as the value to the cell, and it wasn't void at that time, so...?!?

I tried casting the second arg to object like so:

footerMonth1Cell.Value = ReportRunnerConstsAndUtils.SetValueIntOrStr(footerMonth1Cell, object(monthsTruncatedYears[0]));

...but that won't compute, either, with "Invalid expression term 'object'"

...and this similar attempt:

footerMonth1Cell.Value = ReportRunnerConstsAndUtils.SetValueDoubleOrIntOrStr(footerMonth1Cell, monthsTruncatedYears[0] as object);

...elicits, "Cannot implicitly convert type 'void' to 'object'"

Note that the helper method is actually misnamed in the borrowed code; instead of "SetValueIntOrStr" it should be "SetValueDoubleOrIntOrStr"

解决方案

The problem is the line

footerMonth1Cell.Value = SetValueIntOrStr(footerMonth1Cell, monthsTruncatedYears[0]);

SetValueIntOrStr does not return any value (void) and therefore cannot be used to assign a value to the footerMonth1Cell.Value.

This would be valid code because the Value property is already changed inside the function:

SetValueIntOrStr(footerMonth1Cell, monthsTruncatedYears[0]);

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