save to "Assets/Editor" directory

CreateCylinderMesh.cs

using UnityEditor;
using UnityEngine;
using System.Collections;

public class CreateCylinderMesh : MonoBehaviour {

	static int div_x = 40;
	static int div_y = 20;
	static float cylinder_width = 10.0f;
	static float cylinder_height = 5.0f;

	[MenuItem ("Create Object/CylinderMesh")]

	static void Create ()
	{
		GameObject newGameobject = new GameObject ("CylinderMesh");
		
		MeshRenderer meshRenderer = newGameobject.AddComponent<MeshRenderer> ();
		meshRenderer.material = new Material (Shader.Find ("Diffuse"));

		MeshFilter meshFilter = newGameobject.AddComponent<MeshFilter> ();

		meshFilter.mesh = new Mesh ();
		Mesh mesh = meshFilter.sharedMesh;
		mesh.name = "CylinderMesh";

		int w = div_x + 1;
		int h = div_y + 1;

		Vector3 [] vertices = new Vector3[w * h];
		Vector2 [] uv       = new Vector2[w * h];
		int [] triangles    = new int[div_x * div_y * 3 * 2];
		int idx;

		for (int y = 0; y < h; ++y)
		{
			for (int x = 0; x < w; ++x)
			{	
				idx = x + y * w;

				float px = x / (float)div_x;
				float py = 1.0f - y / (float)div_y;

				float r = cylinder_width / 2.0f;
				float th = 2 * Mathf.PI * px;

				float vx = r * -Mathf.Sin(th);
				float vy = cylinder_height * py - cylinder_height / 2;
				float vz = r * -Mathf.Cos(th);

				vertices[idx] = new Vector3(vx, vy, vz);

				float u = px;
				float v = py;
				uv[idx] = new Vector2(u, v);
			}
		}

		idx = 0;
		for (int y = 0; y < div_y; ++y)
		{
			for (int x = 0; x < div_x; ++x)
			{	
				int base_idx = x + y * w;
				triangles[idx ++] = base_idx;
				triangles[idx ++] = base_idx + 1;
				triangles[idx ++] = base_idx + w + 1;
				
				triangles[idx ++] = base_idx + w + 1;
				triangles[idx ++] = base_idx + w;
				triangles[idx ++] = base_idx;
			}
		}

		// まとめて設定するのがポイント
		mesh.vertices = vertices;
		mesh.uv = uv;
		mesh.triangles = triangles;

		mesh.RecalculateNormals ();
		mesh.RecalculateBounds ();
		mesh.Optimize ();

		AssetDatabase.CreateAsset (mesh, "Assets/" + mesh.name + ".asset");
		AssetDatabase.SaveAssets ();

	}
}