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how to create excel custom function using C# visual studio where argument is range

i am trying to use C# xll to make a set of functions for excel. In one function i need to pass range as argument to the function. For eg. say sum of all cells in a range. how to achieve it

The best way to interact with Excel from C# is to use ExcelDNA. If you're using a "C# XLL" then there's a decent chance you ae already using it! Then you need to decide if you want to work with a Range or array (it's the same distinction when writing functions in VBA). If you just want to read values from the sheet, you probably want an array, in which case something like this will work (assuming you've set up ExcelDNA correctly):
public static double SumFunc(double[,] values)
{
int rows = values.GetLength(0);
int cols = values.GetLength(1);
double total = 0;
for (int i = 0; i < rows; i++)
{
for (int j = 0; j < cols; j++)
{
total += values[i, j];
}
}
return total ;
}
Note this will give a #VALUE! error if any of the passed values are not numbers. If you want to use a true Range (actually an ExcelReference in ExcelDNA), you can follow the answers here and here.

C# Excel OutOfMemory exception caused by inserting to a range

I've been working on a quite complex C# VSTO project that does a lot of different things in Excel. However, I have recently stumbled upon a problem I have no idea how to solve. I'm afraid that putting the whole project here will overcomplicate my question and confuse everyone so this is the part with the problem:
//this is a simplified version of Range declaration which I am 100% confident in
Range range = worksheet.Range[firstCell, lastCell]
range.Formula = array;
//where array is a object[,] which basically contains only strings and also works perfeclty fine
The last line that is supposed to insert a [,] array to an Excel range used to work before for smaller Excel books, but now crashes for bigger books with a System.OutOfMemoryException: Insufficient memory to continue the execution of the program and I have no idea why, because it used to work with arrays as long as 500+ elements for one of its dimensions whereas now it crashes for an array with under 400 elements. Furthermore, the RAM usage is about 1.2GB at the moment of crash and I know this project is capable of running perfectly fine with the RAM usage of ~3GBs.
I have tried the following things: inserting this array row by row, then inserting it cell by cell, calling GC.Collect() before each insertion of a row or a cell but it would nonetheless crash with a System.OutOfMemoryException.
So I would appreciate any help in solving this problem or identifying where the error could possibly be hiding, because I can't wrap my head around why it refuses to work for arrays with smaller length (but maybe with slightly bigger contents) at the RAM usage level of 1.2GBs which is like 1/3 of what it used to handle. Thank you!
EDIT
I've been told in the comments that the code above might be too sparse, so here is a more detailed version (I hope it's not too confusing):
List<object[][]> controlsList = new List<object[][]>();
// this list is filled with a quite long method calling a lot of other functions
// if other parts look fine, I guess I'll have to investigate it
int totalRows = 1;
foreach (var control in controlsList)
{
if (control.Length == 0)
continue;
var range = worksheet.GetRange(totalRows + 1, 1, totalRows += control.Length, 11);
//control is an object[n][11] so normally there are no index issues with inserting
range.Formula = control.To2dArray();
}
//GetRange and To2dArray are extension methods
public static Range GetRange(this Worksheet sheet, int firstRow, int firstColumn, int lastRow, int lastColumn)
{
var firstCell = sheet.GetRange(firstRow, firstColumn);
var lastCell = sheet.GetRange(lastRow, lastColumn);
return (Range)sheet.Range[firstCell, lastCell];
}
public static Range GetRange(this Worksheet sheet, int row, int col) => (Range)sheet.CheckIsPositive(row, col).Cells[row, col];
public static T CheckIsPositive<T>(this T returnedValue, params int[] vals)
{
if (vals.Any(x => x <= 0))
throw new ArgumentException("Values must be positive");
return returnedValue;
}
public static T[,] To2dArray<T>(this T[][] source)
{
if (source == null)
throw new ArgumentNullException();
int l1 = source.Length;
int l2 = source[0].Length(1);
T[,] result = new T[l1, l2];
for (int i = 0; i < l1; ++i)
for (int j = 0; j < l2; ++j)
result[i, j] = source[i][j];
return result;
}

I am not 100% sure I figured it out correctly, but it seems like the issue lies within Interop.Excel/Excel limitations and the length of formulas I'm trying to insert: whenever the length approaches 8k characters, which is close to Excel limit for the formula contents, the System.OutOfMemoryException pops out. When I opted to leave lengthy formulas out, the program started working fine.

ExcelWorksheet.UsedRange is counting wrong if file has empty rows on top

I have c# windows application that is reading files content. I wanted to extract values from used rows only.
I am using this code:
int rows = ExcelWorksheet.UsedRange.Rows.Count;
Everything works fine. Except when I have empty rows on top, the counting will be incorrect.
-File has no special characters, formula or such. Just plain text on it.
-The application can read excel xls and xlsx with no issue if the file has no empty rows on top.

Okay, now I've realized I'm doing it all wrong. Of course it will not read all of my UsedRange.Rows because in my for loop, I am starting the reading always on the first row. So I get the ((Microsoft.Office.Interop.Excel.Range)(ExcelWorksheet.UsedRange)).Row; as a starting point of reading
This code works:
int rows = ExcelWorksheet.UsedRange.Rows.Count;
int fRowIndex = ((Microsoft.Office.Interop.Excel.Range)(ExcelWorksheet.UsedRange)).Row;
int rowCycle = 1;
for (int rowcounter = fRowIndex; rowCycle <= rows; rowcounter++)
{
//code for reading
}

Instead of read Excel row-by-row, better to get it in C# as a Range, and then handle it as
Sheet.UsedRange.get_Value()
for whole UsedRange in Sheet. Whenever you'd like to get a part of UsedRange, do it as
Excel.Range cell1 = Sheet.Cells[r0, c0];
Excel.Range cell2 = Sheet.Cells[r1, c1];
Excel.Range rng = Sheet.Range[cell1, cell2];
var v = rng.get_Value();
You well know size of v in C# memory from the values of [r1-r0, c1-c0]

Excel: Losing decimal separator when converting from strings to number

I am trying to read some values from several files and save them in a new .xlsx file with different grouping. I devised a very simple setup to test different formatting and behavior with null values. I always open just-created file in Excel to see outcome. So far no problem.
However in my test-case I can achieve either: A) save the test values as they are (strings) or B) force Excel to regard them as numbers with given format (good), but lose decimal separator (very bad & strange).
I had traced problem to the last line in a code snippet below. The idea of self-assign is from another post somewhere here at SO but right now I am unable to find it.
If the line is commented-out the results are as in a string[,] contents only they are formatted as text (and Excel complains about this with "number formatted as text" message). If I uncomment it, the numbers are regarded as numbers but lose decimal separators. Also the problem might be a fact that I am in Czech Republic and decimal separator is , which might trouble Excel. Moreover, reading the values from start into a double[,] contents is out, since I need to indicate whether value is absent (with empty cell). And double?[,] contents crashes Excel...
Please, haven´t you met this behavior before? I would like to 1) be able to indicate missing value and 2) have contents of cells formatted as a number, not text. Can you help me how to achieve this?
excelApp = new Excel.Application();
excelWorkBooks = excelApp.Workbooks;
excelWorkBook = excelWorkBooks.Add();
excelSheets = excelWorkBook.Sheets;
excelWorkSheet = excelSheets[1]; //Beware! Excel is one-based as opposed to a zero-based C#
string[,] contents = new string[,] { { "1,23", "2,123123123", "3,1415926535" }, { "2,15", null, "" } };
int contentsHeight = contents.GetLength(0);
int contentsWidth = contents.GetLength(1);
System.Globalization.CultureInfo currentCulture = System.Threading.Thread.CurrentThread.CurrentCulture;
string numberFormat = string.Format("0" + currentCulture.NumberFormat.NumberDecimalSeparator + "00E+00");
for (int column = 0; column < contentsWidth; column++) {
excelWorkSheet.Columns[column + 1].NumberFormat = numberFormat;
}
Excel.Range range = excelWorkSheet.Range[excelWorkSheet.Cells[1, 1], excelWorkSheet.Cells[contentsHeight, contentsWidth]];
range.Value = contents;
// range.Value = range.Value; //Problematic place
EDIT: I tryed to change NumberFormat from 0,00E+00 to something like 0,0, 0.0, #,# for the sake of test, but with no success. Either crash (decimal dot) or remains as a text.

There's no need to convert numbers to text before writing them to a cell. Excel understands numbers. A further problem is that the code is trying to set the array as the value of an entire range, as if pasting into Excel.
It's possible to set numbers, even nulls, directly using a simple loop, eg
double?[,] contents = new double?[,] { { 1.23, 2.123123123, 3.1415926535 },
{ 2.15, null, null } };
int contentsHeight = contents.GetLength(0);
int contentsWidth = contents.GetLength(1);
...
for(int i=0;i<= contentsHeight; i++)
for (int j = 0; j <= contentsWidth; j++)
excelWorkSheet.Cells[i+1,j+1].Value = contents[i,j];
Instead of using Excel through Interop though, it's better to use a package like EPPlus to generate xlsx files directly without having Excel installed. This allows generating real Excel files even on web servers, where installing Excel is impossible.
The code for this particular problem would be similar:
var file = new FileInfo("test.xlsx");
using (var pck = new ExcelPackage(file))
{
var ws = pck.Workbook.Worksheets.Add("Rules");
for(int i=0;i<= contentsHeight; i++)
for (int j = 0; j <= contentsWidth; j++)
ws.Cells[i+1,j+1].Value = contents[i,j];
pck.Save();
}
EPPlus has some convenience methods that make loading a sheet easy, eg LoadFromDataTable or LoadFromCollection. If the data came from a DataTable, creating the sheet would be as simple as:
var file = new FileInfo("test.xlsx");
using (var pck = new ExcelPackage(file))
{
var ws = pck.Workbook.Worksheets.Add("Rules");
ws.LoadFromDataTable(myTable);
pck.Save();
}
LoadFromDataTable returns an ExcelRange which allows cell formatting just like Excel Interop.

Excel Interop Coloring Cell Using Range

I need to color Excel cells in a fast manner. I found similar method to write to Excel cells which for me is really fast, so I tried applying the same method when coloring the cells. Consider the following code:
xlRange = xlWorksheet.Range["A6", "AS" + dtSchedule.Rows.Count];
double[,] colorData = new double[dtSchedule.Rows.Count, dtSchedule.Columns.Count];
for (var row = 0; row < dtSchedule.Rows.Count; row++)
{
for (var column = 0; column < dtSchedule.Columns.Count; column++)
{
if (column <= 3)
{
colorData[row, column] = GetLightColor2("#ffffff");
continue;
}
if (dtSchedule.Rows[row][column].ToString() != "#000000" && !string.IsNullOrEmpty(dtSchedule.Rows[row][column].ToString()))
{
string[] schedule = dtSchedule.Rows[row][column].ToString().Split('/');
string color = schedule[0].Trim();
colorData[row, column] = GetLightColor2(color);
continue;
}
colorData[row, column] = GetLightColor2("#000000");
}
}
xlRange.Interior.Color = colorData;
This is the GetLightColor2 function:
private double GetLightColor2(string hex)
{
return ColorTranslator.ToOle(ColorTranslator.FromHtml(hex));
}
When I ran the code, an error was thrown at
xlRange.Interior.Color = colorData;
With the following error:
System.Runtime.InteropServices.COMException (0x80020005): Type
mismatch. (Exception from HRESULT: 0x80020005 (DISP_E_TYPEMISMATCH))
at System.RuntimeType.ForwardCallToInvokeMember(String memberName,
BindingFlags flags, Object target, Int32[] aWrapperTypes, MessageData&
msgData) at Microsoft.Office.Interop.Excel.Interior.set_Color(Object
value)
I could not find any other workaround unless coloring the cell by looping through each cell which is really slow. Or is it that I'm doing it the wrong way.
Thank you for your kind attention guys.

If your question is not about excel addin, I would strongly recommend to follow Akhil R J's advice. It's not the last big problem you'll encounter in interop, this technology is just one big problem and bug. If for some reason you cannot, I can tell you some things about your problem:
1) There is no way to do what you want using arrays. It is possible only for values and formulas.
2) Set Application.ScreenUpdating = false, when you set colors or any other operation with excel. Then it freezes user input, things go faster.
3) If many cells have the same color - use Application.Union to make a range from separated cells of the same color. But it's effective only to merge up to 50 cells in one time. If you take more, merging operation takes too much time, and it's not effective. After that, just set one color to the whole merged range. Pretty effective, around 5-10 times faster in my case.
4) There is another way, difficult one. I going to try it myself for the same problem (I have an addin, so I cannot just start to use OpenXML). Using interop, you can copy the target range to the windows clipboard. In the clipboard it is stored in many formats, including something OpenXMl-like. So you can edit it in the clipboard and paste back, using interop again. I think it's the fastest way, but it must be very time consuming to write this code.