Added custom exception to replace all the 'return null' statements

TEST_DecodePng/Form1.cs

@@ -80,7 +80,14 @@ namespace TEST_DecodePng
         private void btnDecode_Click(object sender, EventArgs e)
         {
             PngDecoder decoder = new PngDecoder();
+            try{
                 decoder.Decode(selectedFile);
             }
+            catch(Exception ex)
+            {
+                MessageBox.Show(ex.Message);
+            }
+            
+        }
     }
 }

TEST_DecodePng/InvalidPngHeadersFileException.cs

@@ -0,0 +1,24 @@
+using System;
+using System.Collections.Generic;
+using System.Linq;
+using System.Text;
+using System.Threading.Tasks;
+
+namespace TEST_DecodePng
+{
+    public class InvalidPngHeadersFileException : Exception
+    {
+        public InvalidPngHeadersFileException()
+        {
+            //empty
+        }
+        public InvalidPngHeadersFileException(string message):base(message)
+        {
+            //empty
+        }
+        public InvalidPngHeadersFileException(string message, Exception inner):base(message,inner)
+        {
+            //empty
+        }
+    }
+}

TEST_DecodePng/PngDecoder.cs

@@ -48,7 +48,7 @@ namespace TEST_DecodePng
 
             //Checks if the file has a valid png signature
             if (!compareByteArrays(EXPECTED_PNG_SIGNATURE, pngSignature))
-                return null;
+                throw (new InvalidPngHeadersFileException("File signature not recognized"));
 
             //We are now sure that we are working with a png file and we will then read chuncks
 
@@ -70,7 +70,7 @@ namespace TEST_DecodePng
 
             //If the first chunck is not the IHDR that means that we should not continue to decompress it
             if (ckType != FIRST_CHUNK_TYPE)
-                return null;
+                throw (new InvalidPngHeadersFileException("First chunck not valid, possible corruption of the file"));
 
             //As it is the first chunck, his structure is a little bit different
 
@@ -112,41 +112,43 @@ namespace TEST_DecodePng
             //Those are all the rules seen on the documentation at : https://www.w3.org/TR/PNG-Chunks.html
 
             if (compressionMethod != 0 || filterMethod != 0 || interlaceMethod != 0 && interlaceMethod != 1)
-                return null;
+                throw (new InvalidPngHeadersFileException("Incoherent IHDR chunck informations"));
+
+            InvalidPngHeadersFileException incoherentColorsAndBitDepths =  new InvalidPngHeadersFileException("Incoherent colorType and bitDepths, could be due to corruption of the file");
             switch (colorType)
             {
                 case 0:
                     //Each pixel is a grayscale sample.
                     if (bitDepth != 1 && bitDepth != 2 && bitDepth != 4 && bitDepth != 8 && bitDepth != 16)
-                        return null;
+                        throw (incoherentColorsAndBitDepths);
                     break;
                 case 2:
                     //Each pixel is an R,G,B triple.
                     if (bitDepth != 8 && bitDepth != 16)
-                        return null;
+                        throw (incoherentColorsAndBitDepths);
                     break;
                 case 3:
                     //Each pixel is a palette index, a PLTE chunk must appear.
                     if (bitDepth != 1 && bitDepth != 2 && bitDepth != 4 && bitDepth != 8)
-                        return null;
+                        throw (incoherentColorsAndBitDepths);
                     break;
                 case 4:
                     //Each pixel is a grayscale sample, followed by an alpha sample.
                     if (bitDepth != 8 && bitDepth != 16)
-                        return null;
+                        throw (incoherentColorsAndBitDepths);
                     break;
                 case 6:
                     //Each pixel is an R,G,B triple, followed by an alpha sample.
                     if (bitDepth != 8 && bitDepth != 16)
-                        return null;
+                        throw (incoherentColorsAndBitDepths);
                     break;
                 default:
                     //Only 0,2,3,4 and 6 are valid color type values
-                    return null;
+                        throw (incoherentColorsAndBitDepths);
                     break;
             }
 
-
+            //Insert code
 
             MessageBox.Show("Image width: " + imageWidth +
                 Environment.NewLine + "Image height: " + imageHeight +
@@ -165,16 +167,6 @@ namespace TEST_DecodePng
             //TO REMOVE
             return new Bitmap(100, 100);
         }
-        public byte[] ReadChunck()
-        {
-            //Must always return two values in the array or null
-
-
-
-            //TO REMOVE
-            return new byte[] { 0, 0 };
-        }
-
         public bool compareByteArrays(byte[] firstArray, byte[] secondArray)
         {
             if (firstArray.Length != secondArray.Length)

TEST_DecodePng/TEST_DecodePng.csproj

@@ -52,6 +52,7 @@
     <Compile Include="Form1.Designer.cs">
       <DependentUpon>Form1.cs</DependentUpon>
     </Compile>
+    <Compile Include="InvalidPngHeadersFileException.cs" />
     <Compile Include="PngDecoder.cs" />
     <Compile Include="Program.cs" />
     <Compile Include="Properties\AssemblyInfo.cs" />