I have taken my dinner ,( so I can't join you for dinner) >>this is best way to say . I have eaten my dinner >The same but it not as common as above. I finished my dinner > Here you are just telling that you just completed eating your dinner. Do not finish the dinner that you are having now because I want to take you out for a meal ,but if you have taken a heavy lunch, then I hope you can eat at least some food .

they are actually 2 seperate things... eating and finishing...

Maybe i just have my dinner if iam not mistaken

Thanks Mary!

I already ate.