Single Cube Generator

using System.Collections.Generic;
using UnityEngine;

public class SingleCubeGenerator : MonoBehaviour
{
    [SerializeField] MeshFilter _meshFilter = null;

    // we use the values 24 and 36 because there are 6 * 4 vertices (24) and 6 * 6 triangle indices (36)
    private Vector3[] _vertices = new Vector3[24];
    private Vector3[] _normals = new Vector3[24];
    private Vector2[] _uvs = new Vector2[24];
    private int[] _triangles = new int[36];

    void Start()
    {
        PopulateBuffers();

        if (_meshFilter.sharedMesh == null)
            _meshFilter.sharedMesh = new Mesh();
        var mesh = _meshFilter.sharedMesh;
        // we clear in case this is an existing mesh.
        mesh.Clear();
        // we use the Set* methods because they are less heavy than the normal properties!
        mesh.SetVertices(_vertices);
        mesh.SetNormals(_normals);
        mesh.SetUVs(0, _uvs);
        mesh.SetTriangles(_triangles, 0);
    }

    void PopulateBuffers()
    {
        // Set some counters to use in the CreateQuad method
        var vertexCount = 0;
        var triangleCount = 0;
        // Top face
        CreateQuad(
            new Vector3(-.5f, .5f, -.5f),
            new Vector3(-.5f, .5f, .5f),
            new Vector3(.5f, .5f, .5f),
            new Vector3(.5f, .5f, -.5f),
            Vector3.up,
            ref vertexCount,
            ref triangleCount);
        // Bottom face
        CreateQuad(
            new Vector3(.5f, -.5f, -.5f),
            new Vector3(.5f, -.5f, .5f),
            new Vector3(-.5f, -.5f, .5f),
            new Vector3(-.5f, -.5f, -.5f),
            Vector3.down,
            ref vertexCount,
            ref triangleCount);
        // Front face
        CreateQuad(
            new Vector3(.5f, -.5f, .5f),
            new Vector3(.5f, .5f, .5f),
            new Vector3(-.5f, .5f, .5f),
            new Vector3(-.5f, -.5f, .5f),
            Vector3.forward,
            ref vertexCount,
            ref triangleCount);

        // todo: implement remaining faces
    }

    void CreateQuad(Vector3 a, Vector3 b, Vector3 c, Vector3 d, Vector3 normal, ref int vertexCount, ref int triangleCount)
    {
        _vertices[vertexCount + 0] = a;
        _vertices[vertexCount + 1] = b;
        _vertices[vertexCount + 2] = c;
        _vertices[vertexCount + 3] = d;

        _normals[vertexCount + 0] = normal;
        _normals[vertexCount + 1] = normal;
        _normals[vertexCount + 2] = normal;
        _normals[vertexCount + 3] = normal;

        _uvs[vertexCount + 0] = new Vector2(0, 0);
        _uvs[vertexCount + 1] = new Vector2(0, 1);
        _uvs[vertexCount + 2] = new Vector2(1, 1);
        _uvs[vertexCount + 3] = new Vector2(1, 0);

        _triangles[triangleCount + 0] = vertexCount + 0;
        _triangles[triangleCount + 1] = vertexCount + 1;
        _triangles[triangleCount + 2] = vertexCount + 2;
        _triangles[triangleCount + 3] = vertexCount + 2;
        _triangles[triangleCount + 4] = vertexCount + 3;
        _triangles[triangleCount + 5] = vertexCount + 0;

        vertexCount += 4;
        triangleCount += 6;
    }
}