Many a times we have a scenario when we have to read from a file write the dirty row into a text file.
In my case, I had to create the text file for each run hence I created a file with 
dynamic name.
SSIS would create the file even when there is no error.
Now we need to delete the file in case the file is empty i.e. there are no error rows.
As we all know 
rowcount task come in hand to calculate the no. of records which error out.

As the name of the error file is dynamic, we use the 
EvaluateAsExpression property of the variable.

As you can see in the screen short above we have a variable name
DataErrorFilePathEvaluated and the property EvaluateAsExpression is set to true. The expression value is set to 
append datetime to it.
The variable 
DataErrorFilePathEvaluated is also used as the 
ConnectionString of the output error file and same is used as the 
ConnectionString for the 
file system task to delete the file in case no records error out.

Hope it Helps :)