how to draw a line in excel created by c#

Okay. Here is the thing.

Once you have the worksheet reference. You can code below. (You can find the shapre enumeration here -> https://msdn.microsoft.com/en-us/library/office/ff862770.aspx?f=255&MSPPError=-2147217396)

workSheet.Shapes.AddShape(Microsoft.Office.Core.MsoAutoShapeType.msoShapeLineInverse,50,50,55,2);

or else

workSheet.Shapes.AddShape(Microsoft.Office.Core.MsoAutoShapeType.msoShapeLineCallout1NoBorder,5,5,55,2);

You should see an output something like this.

Please mark it as answer if it resolves the issue.