Distinctcount GroupBy?

or if you want the DAX version:

Table = 
ADDCOLUMNS(
    SUMMARIZECOLUMNS( 
        'Fact'[ProjectName ], 
        'Fact'[Dep      ]
    ),
    "Distinct Count", 
    CALCULATE(
        DISTINCTCOUNT('Fact'[WorkerCode])
    )
)

I'll quickly note that if you're goal is to simply have a visual that shows this info in a report/dashboard, then you don't need to write any DAX.

Simply create a table visual, add in ProjectName, Dep, and WorkerCode, then hit dropdown on WorkerCode value and change aggregation setting to "Count (Distinct)" – you can change display name of the columns in the same dropdown menu, too (if for example you don't like "Count of WorkerCode").