Measure that depends of the context + always exclude one value

[jct999]

Hello,

I have a fact table with a few columns. Among them :

• A column contains Categories : A, B, C or null
• A column contains Quantity, that are integer.

I want to create a measure that calculates the sum of the Quantity, with the following rules : 

• If there is no filter/slicer context for the Category column in the report, add the quantity for all non-null categories.
• If there is a filter/slicer context for the Category column in the report, add the quantity for the selected categories except null
  
  

In other word, the measure add quantity depending of the filter context, but null-category is always excluded.

What is the best way to do that in DAX ?



Thanks