Dynamic SQL Syntax H_ll Please Help

This forum has helped me get to this point a couple times with this project BUT I just can't seem to get the syntax correct. Either one of these two statements do exactly what I want them to do:
SELECT @.RtnValue = Column0 FROM MyTable WHERE RowIndex = @.RIndex
SELECT @.RtnValue = (SELECT Column0 FROM MyTable WHERE RowIndex = @.RIndex)

@.RtnValue is the value the program will work with.

The only problem is that Column0 and @.RIndex need to be dynamic so I can index through each Column and Row of the table.

This is the code I am trying to use to do this dynamically, naturally it will be in two while loop to index through each Column and Row

DECLARE @.RtnValue smallint
SET @.RtnValue = 0

DECLARE @.RIndex smallint
SET @.RIndex = 20

DECLARE @.ColumnName varchar(10)
SET @.ColumnName = 'Column0'

DECLARE @.MySelectString varchar(200)
SET @.MySelectString0 = 'SELECT @.RtnValue = ( SELECT ' + @.ColumnName + ' FROM MyTable WHERE RowIndex = ' + @.RIndex + ' )'
--SET @.MySelectString1 = 'SELECT @.RtnValue = ( SELECT ' + @.ColumnName + ' FROM MyTable WHERE RowIndex = 1 )'

EXEC( @.MySelectString0 )
--EXEC( @.MySelectString1 )

@.MySelectString0 produces this error:
Server: Msg 245, Level 16, State 1, Line 23
Syntax error converting the varchar value 'SELECT @.RtnValue = ( SELECT Column0 FROM MyTable WHERE RowIndex = ' to a column of data type smallint.

@.MySelectString1 produces this error:
Server: Msg 137, Level 15, State 1, Line 1
Must declare the variable'@.RtnValue'.

I have tried many different combinations of syntax but can not seem to get the magic combination. Can someone tell me the correct syntax to get this to work.

Thank you in advance.

try something like this :

SET @.MySelectString0 = 'SELECT @.RtnValue = ( SELECT ' + @.ColumnName + ' FROM MyTable WHERE RowIndex = ' + CONVERT(VARCHAR,@.RIndex) + ' )'

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Thank You for your responce I will try your solution. I sure hope it works I have been stuck on this problem to long Thanks again