add unit tests for null handling, fix assertion in rust code

crates/servicepoint_binding_c/src/cp437_grid.rs

@@ -70,7 +70,7 @@ pub unsafe extern "C" fn sp_cp437_grid_load(
     data: *const u8,
     data_length: usize,
 ) -> NonNull<SPCp437Grid> {
-    assert!(data.is_null());
+    assert!(!data.is_null());
     let data = std::slice::from_raw_parts(data, data_length);
     let result = Box::new(SPCp437Grid(servicepoint::Cp437Grid::load(
         width, height, data,

crates/servicepoint_binding_cs/ServicePoint.Tests/BitVecTests.cs

@@ -0,0 +1,12 @@
+namespace ServicePoint.Tests;
+
+public class BitVecTests
+{
+    [Fact]
+    public void UseAfterFree()
+    {
+        var bitvec = new BitVec(8);
+        _ = Command.BitmapLinear(0, bitvec, CompressionCode.Uncompressed);
+        Assert.Throws<NullReferenceException>(() => _ = Command.BitmapLinear(0, bitvec, CompressionCode.Uncompressed));
+    }
+}

crates/servicepoint_binding_cs/ServicePoint.Tests/BitmapTests.cs

@@ -0,0 +1,12 @@
+namespace ServicePoint.Tests;
+
+public class BitmapTests
+{
+    [Fact]
+    public void UseAfterFree()
+    {
+        var bitmap = Bitmap.NewScreenSized();
+        _ = Command.BitmapLinearWin(0, 0, bitmap, CompressionCode.Uncompressed);
+        Assert.Throws<NullReferenceException>(() => _ = Command.BitmapLinearWin(0, 0, bitmap, CompressionCode.Uncompressed));
+    }
+}

crates/servicepoint_binding_cs/ServicePoint.Tests/BrightnessGridTests.cs

@@ -0,0 +1,12 @@
+namespace ServicePoint.Tests;
+
+public class BBrightnessGridTests
+{
+    [Fact]
+    public void UseAfterFree()
+    {
+        var grid = new BrightnessGrid(23, 42);
+        _ = Command.CharBrightness(0, 0, grid);
+        Assert.Throws<NullReferenceException>(() => _ = Command.CharBrightness(0, 0, grid));
+    }
+}

crates/servicepoint_binding_cs/ServicePoint.Tests/CommandTests.cs

@@ -5,10 +5,11 @@ public class CommandTests
     private Connection _fakeConnection = Connection.Fake();
 
     [Fact]
-    public void Test1()
+    public void UseAfterSend()
     {
         var command = Command.Clear();
         _fakeConnection.Send(command);
         Assert.Throws<NullReferenceException>(() => _fakeConnection.Send(command));
+        _fakeConnection.Send(Command.Clear());
     }
 }

crates/servicepoint_binding_cs/ServicePoint.Tests/Cp437GridTests.cs

@@ -0,0 +1,32 @@
+using System.Runtime.CompilerServices;
+
+namespace ServicePoint.Tests;
+
+public class Cp437GridTests
+{
+    [Fact]
+    public void UseAfterFree()
+    {
+        var grid = new Cp437Grid(2, 3);
+        _ = Command.Cp437Data(0, 0, grid.Clone());
+        _ = Command.Cp437Data(0, 0, grid);
+        Assert.Throws<NullReferenceException>(() => _ = Command.Cp437Data(0, 0, grid));
+    }
+
+    [Fact]
+    public void ReadAndWriteString()
+    {
+        var grid = new Cp437Grid(3, 2);
+        grid[1] = "abc";
+        Assert.Equal("abc", grid[1]);
+    }
+
+    [Fact]
+    public void LoadSpan()
+    {
+        var ascii_str = "abc123"u8;
+        var grid = Cp437Grid.Load(3, 2, ascii_str);
+        Assert.Equal("abc", grid[0]);
+        Assert.Equal("123", grid[1]);
+    }
+}