MSVC + DLL + Ziglang

build.zig

const std = @import("std");

pub fn build(b: *std.Build) void {
    const target = b.standardTargetOptions(.{});
    const optimize = b.standardOptimizeOption(.{});

    const lib_mod = b.createModule(.{
        .root_source_file = b.path("lib/mylib/mylib.zig"),
        .target = target,
        .optimize = optimize,
        .link_libc = true,
    });

    const exe_mod = b.createModule(.{
        .root_source_file = b.path("src/main.zig"),
        .target = target,
        .optimize = optimize,
    });

    exe_mod.addImport("mylib", lib_mod);

    const exe = b.addExecutable(.{
        .name = "using_dll_msvc",
        .root_module = exe_mod,
        .linkage = .dynamic,
    });

    exe.addLibraryPath(b.path("./lib/mylib/windows"));
    exe.linkSystemLibrary("mylib");

    // to copy the dll to the zig-out directory
    b.installBinFile("./lib/mylib/windows/mylib.dll", "mylib.dll");

    b.installArtifact(exe);

    const run_cmd = b.addRunArtifact(exe);

    run_cmd.step.dependOn(b.getInstallStep());

    if (b.args) |args| {
        run_cmd.addArgs(args);
    }

    const run_step = b.step("run", "Run the app");
    run_step.dependOn(&run_cmd.step);
}

main.zig

const std = @import("std");

const mylib = @import("mylib");

pub fn main() !void {
    std.debug.print("add(1, 2) = {d}\n", .{mylib.add(1, 2)});
    std.debug.print("add(10, 20) = {d}\n", .{mylib.add(10, 20)});
    std.debug.print("add(10, 20) = {d}\n", .{mylib.add(10, 21)});
    std.debug.print("add(10, 20) = {d}\n", .{mylib.add(555, 999)});
}

// $ zig build run --summary all
// add(1, 2) = 3
// add(10, 20) = 30
// add(10, 20) = 31
// add(10, 20) = 1554

make.cmd

@echo off

REM create a .obj
cl /nologo /c mylib.c

REM create a .lib
lib /nologo /OUT:mylib.lib mylib.obj

REM create a .dll
link /nologo /DLL /OUT:mylib.dll mylib.obj