ISNULL (SSIS Expression)
Applies to: 
SQL Server
SSIS Integration Runtime in Azure Data Factory
Returns a Boolean result based on whether an expression is null.
Syntax
ISNULL(expression)
Arguments
expression
Is a valid expression of any data type.
Result Types
DT_BOOL
Expression Examples
This example returns TRUE if the DiscontinuedDate column contains a null value.
ISNULL(DiscontinuedDate)
This example returns "Unknown last name" if the value in the LastName column is null, otherwise it returns the value in LastName.
ISNULL(LastName)? "Unknown last name":LastName
This example always returns TRUE if the DaysToManufacture column is null, regardless of the value of the variable AddDays.
ISNULL(DaysToManufacture + @AddDays)