Why am I getting, “Cannot implicitly convert type 'void' to 'object'” with this code (EPPlus)?

问题

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"

回答1:

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]);

来源：https://stackoverflow.com/questions/39417362/why-am-i-getting-cannot-implicitly-convert-type-void-to-object-with-this

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