Hiya Amon, I dont think theres an equivalent DefData setup in C#. I'm assuming you want to fill an array with the data, which can be done like this:-
int [] intArray;
intArray = new int[3] {0, 1, 2};
So imagine that being a tilemap you want to initialize, you can do this:-
int [] intArray;
intArray = new int[77] {
1,1,1,1,1,1,1,1,1,1,1
1,0,0,0,0,0,0,0,0,0,1
1,0,0,0,0,0,0,0,0,0,1
1,0,0,0,0,1,0,0,0,0,1
1,0,0,0,0,0,0,0,0,0,1
1,0,0,0,0,0,0,0,0,0,1
1,1,1,1,1,1,1,1,1,1,1};
To add extra maps, just add more dimensions to the array and fill it up like so:-
int[,] multiArray = new int[3, 2] { {1, 2}, {3, 4}, {5, 6} };
Hope that helps