How to un-hide all columns in a smartsheet using smartsheet C# SDK?
The hide attribute on the column object can be read to see the current status of a column and if it is hidden or not, but it currently it isn't writable to allow changing its state. Using the Smartsheet API you can see the current status of columns and if they are hidden or not, but you aren't able to adjust the value to hide or unhide columns. That would need to be done manually in Smartsheet in your browser.
You might check out the Smartsheet API documentation. There are examples for updating columns.
Just update the column attribute 'hidden' to False. I'm not into c#. But see the screenshot for Python. The documentation has a similar example for c#.