declare @myvariable as varchar, @len1 varchar, @len2 varchar
set @myvariable = 'hi hello how are you'
set @len1 = (select datalength(cast(@myvariable as char)))
print @len1
set @len1 = (select datalength(cast(@myvariable as varchar)))
print @len1
set @len2 = (select datalength(convert(char, @myvariable)))
print @len2
set @len2 = (select datalength(convert(varchar, @myvariable)))
print @len2

*
1
*
1