public override bool IsEmpty {get;}

// create the multipatchBuilderEx object
var builder = new ArcGIS.Core.Geometry.MultipatchBuilderEx(multipatch);

// check some properties
bool hasM = builder.HasM;
bool hasID = builder.HasID;
bool isEmpty = builder.IsEmpty;
bool hasNormals = builder.HasNormals;

var patches = builder.Patches;
int patchCount = patches.Count;

// if there's some patches
if (patchCount > 0)
{
  int pointCount = builder.GetPatchPointCount(0);

  // replace the first point in the first patch
  if (pointCount > 0)
  {
    // get the first point
    var pt = builder.GetPoint(0, 0);
    builder.SetPoint(0, 0, newPoint);
  }

  // check which patches currently contain the texture
  var texture = builder.QueryPatchIndicesWithTexture(brickTextureResource);

  // assign a texture material
  patches[0].Material = brickMaterialTexture;
}

// update the builder for M awareness
builder.HasM = true;
// synchronize the patch attributes to match the builder attributes
//   in this instance because we just set HasM to true on the builder, each patch will now get a default M value for it's set of coordinates
builder.SynchronizeAttributeAwareness();

// call ToGeometry to get the multipatch
multipatch = builder.ToGeometry() as Multipatch;

// multipatch.HasM will be true