. Net using Com components

Some time ago. when the Supermap component (activeX) is used in net, it is often found that the memory cannot be written. Later, it contacted the super map company to say that the Com object needs to be released in the code.
Today, when using the MapX component, we found another strange problem: NullReferenceException occurs after processing part of the data during topology processing. The Code is as follows:

For (int I = 1; I <= axMap1.Layers [LayerName]. AllFeatures. Count; I ++)
{
MapXLib. Feature ft = axMap1.Layers [LayerName]. AllFeatures [I];
X1 = ft. Parts [1]. get_X (1 );
Y1 = ft. Parts [1]. get_Y (1 );
ProcessData (x1, y1 );
}
After checking for half a day, I did not find the cause. Later I thought it may be related to the release of the Com object, so I adjusted the code:
MapXLib. Layer lay = axMap1.Layers [LayerName];
MapXLib. Features fs = lay. AllFeatures;
Int c = fs. Count;
For (int I = 1; I <= c; I ++)
{
MapXLib. Feature ft = fs. _ Item (I );
MapXLib. Parts pts = ft. Parts;
MapXLib. Points pss = pts [1];
X1 = pss. get_X (1 );

Y1 = pss. get_Y (1 );

ProcessData (x1, y1 );
System. Runtime. InteropServices. Marshal. ReleaseComObject (pss );
Pss = null;
System. Runtime. InteropServices. Marshal. ReleaseComObject (pts );

Pts = null;
System. Runtime. InteropServices. Marshal. ReleaseComObject (ft );

Ft = null;
}
System. Runtime. InteropServices. Marshal. ReleaseComObject (fs );
Fs = null;
System. Runtime. InteropServices. Marshal. ReleaseComObject (lay );
Lay = null;

The adjusted code runs normally.

Several notes are found during the adjustment:
1. As long as the Com object is defined, it must be released after use. Otherwise, problems will occur after multiple calls.
2. Temporary variables also need to be released. For example
X1 = ft. Parts [1]. get_X (1 );
Discount:
MapXLib. Parts pts = ft. Parts;

MapXLib. Points pss = pts [1];

X1 = pss. get_X (1 );

Y1 = pss. get_Y (1 );
Release the variable.
3. If a method returns a Com Object, define the returned variable and release it.

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