D, D Source Code File (.d)
The D Developer file format
The D file extension is used for files that store code written in a special programming language called D. D is similar to the languages C++ and C#, Java and Eiffel. The file itself can be opened by any text editing program since the source code is saved in a plain text format. However, to execute the D file, a special program is needed.
Technical details of D files
Source code contained in the D file is stored as plain text, making it possible for every text editor to open, view and save files in the D format. D is an object-oriented, imperative programming language that allows multiple paradigms. To execute D files, they have to be run with a special D compiler.