broke out merge logic and added UnitTests

MergePDF.sln

@@ -5,6 +5,8 @@ VisualStudioVersion = 15.0.27130.2026
 MinimumVisualStudioVersion = 10.0.40219.1
 Project("{FAE04EC0-301F-11D3-BF4B-00C04F79EFBC}") = "MergePDF", "MergePDF\MergePDF.csproj", "{8BF6B775-F54A-4590-959B-FD6D5AEC1280}"
 EndProject
+Project("{FAE04EC0-301F-11D3-BF4B-00C04F79EFBC}") = "MergePDFTests", "MergePDFTests\MergePDFTests.csproj", "{116308E4-C514-461A-B222-AEC05F1DFEFC}"
+EndProject
 Global
 	GlobalSection(SolutionConfigurationPlatforms) = preSolution
 		Debug|Any CPU = Debug|Any CPU
@@ -15,6 +17,10 @@ Global
 		{8BF6B775-F54A-4590-959B-FD6D5AEC1280}.Debug|Any CPU.Build.0 = Debug|Any CPU
 		{8BF6B775-F54A-4590-959B-FD6D5AEC1280}.Release|Any CPU.ActiveCfg = Release|Any CPU
 		{8BF6B775-F54A-4590-959B-FD6D5AEC1280}.Release|Any CPU.Build.0 = Release|Any CPU
+		{116308E4-C514-461A-B222-AEC05F1DFEFC}.Debug|Any CPU.ActiveCfg = Debug|Any CPU
+		{116308E4-C514-461A-B222-AEC05F1DFEFC}.Debug|Any CPU.Build.0 = Debug|Any CPU
+		{116308E4-C514-461A-B222-AEC05F1DFEFC}.Release|Any CPU.ActiveCfg = Release|Any CPU
+		{116308E4-C514-461A-B222-AEC05F1DFEFC}.Release|Any CPU.Build.0 = Release|Any CPU
 	EndGlobalSection
 	GlobalSection(SolutionProperties) = preSolution
 		HideSolutionNode = FALSE

MergePDF/App.config

@@ -1,6 +1,6 @@
-<?xml version="1.0" encoding="utf-8" ?>
+<?xml version="1.0" encoding="utf-8"?>
 <configuration>
     <startup> 
-        <supportedRuntime version="v4.0" sku=".NETFramework,Version=v4.6.1" />
+        <supportedRuntime version="v4.0" sku=".NETFramework,Version=v4.7.2"/>
     </startup>
-</configuration>
+</configuration>

MergePDF/Form1.cs

@@ -20,10 +20,7 @@ public partial class Form1 : Form
         private List<TextBox> Textboxes { get; set; }
         private List<OpenFileDialog> FileDialogs { get; set; }
 
-        private List<string> Documents { get; set; }
-
-        // Create a document for the merged result.
-        private PdfDocument outDocument { get; set; }
+        private PdfMerger _pdfMerger;
 
         private OpenFileDialog openFileDialog { get; set; }
         private SaveFileDialog saveFileDialog { get; set; }
@@ -35,7 +32,7 @@ public Form1()
             Buttons = new List<Button>();
             Textboxes = new List<TextBox>();
             FileDialogs = new List<OpenFileDialog>();
-            Documents = new List<string>();
+            _pdfMerger = new PdfMerger();
             CreateOpenFileDialog();
             CreateSaveFileDialog();
         }
@@ -51,33 +48,11 @@ private void richConsole_Enter(object sender, EventArgs e)
             ActiveControl = null;
         }
 
-        private void CopyPages(PdfDocument from, PdfDocument to)
-        {
-            for (int i = 0; i < from.PageCount; i++)
-            {
-                to.AddPage(from.Pages[i]);
-            }
-        }
-
         private void btnMergePdf_Click(object sender, EventArgs e)
         {
-
-            outDocument = new PdfDocument();
-
             try
             {
-                using (PdfDocument outPdf = new PdfDocument())
-                {
-                    foreach (var document in Documents)
-                    {
-                        document.Replace(@"\", "/");
-                        PdfDocument importPdf = PdfReader.Open(document, PdfDocumentOpenMode.Import);
-                        CopyPages(importPdf, outPdf);
-                    }
-                    var outputLocation = saveFileDialog.FileName.Replace(@"\", "/");
-                    outPdf.Save(outputLocation);
-                }
-
+                _pdfMerger.MergeFiles(saveFileDialog.FileName);
                 richConsole.AppendText(string.Format("Merged successfully!\n"));
             }catch(Exception ex)
             {
@@ -126,7 +101,7 @@ private void RemoveMergeRows()
             Buttons.Clear();
             Labels.Clear();
             Textboxes.Clear();
-            Documents.Clear();
+            _pdfMerger.Clear();
         }
 
         private void CreateOpenFileDialog()
@@ -199,7 +174,7 @@ private Button CreateOpenButton(int yOffset, int increment)
                     try
                     {
                         string fileName = dialog.FileName;
-                        Documents.Add(fileName);
+                        _pdfMerger.AddFile(fileName);
                         Textboxes[increment].Text = fileName;
                     }
                     catch(System.IO.IOException)

MergePDF/MergePDF.csproj

@@ -9,7 +9,7 @@
     <OutputType>WinExe</OutputType>
     <RootNamespace>MergePDF</RootNamespace>
     <AssemblyName>MergePDF</AssemblyName>
-    <TargetFrameworkVersion>v4.6.1</TargetFrameworkVersion>
+    <TargetFrameworkVersion>v4.7.2</TargetFrameworkVersion>
     <FileAlignment>512</FileAlignment>
     <AutoGenerateBindingRedirects>true</AutoGenerateBindingRedirects>
     <IsWebBootstrapper>false</IsWebBootstrapper>
@@ -29,6 +29,7 @@
     <BootstrapperEnabled>true</BootstrapperEnabled>
     <NuGetPackageImportStamp>
     </NuGetPackageImportStamp>
+    <TargetFrameworkProfile />
   </PropertyGroup>
   <PropertyGroup Condition=" '$(Configuration)|$(Platform)' == 'Debug|AnyCPU' ">
     <PlatformTarget>AnyCPU</PlatformTarget>
@@ -81,6 +82,7 @@
     <Compile Include="Form1.Designer.cs">
       <DependentUpon>Form1.cs</DependentUpon>
     </Compile>
+    <Compile Include="PdfMerger.cs" />
     <Compile Include="Program.cs" />
     <Compile Include="Properties\AssemblyInfo.cs" />
     <EmbeddedResource Include="Form1.resx">
@@ -94,6 +96,7 @@
     <Compile Include="Properties\Resources.Designer.cs">
       <AutoGen>True</AutoGen>
       <DependentUpon>Resources.resx</DependentUpon>
+      <DesignTime>True</DesignTime>
     </Compile>
     <None Include="packages.config">
       <SubType>Designer</SubType>

MergePDF/PdfMerger.cs

@@ -0,0 +1,78 @@
+using System;
+using System.Collections.Generic;
+using System.Linq;
+using System.Text;
+using System.Threading.Tasks;
+using PdfSharp.Pdf;
+using PdfSharp.Pdf.IO;
+
+namespace MergePDF
+{
+    public class PdfMerger
+    {
+        public List<string> Documents { get; set; }
+        public string OutputFile { get; set; }
+
+        public PdfMerger()
+        {
+            Init();
+        }
+
+        private void Init()
+        {
+            Documents = new List<string>();
+            OutputFile = string.Empty;
+        }
+
+        public void Clear()
+        {
+            Init();
+        }
+
+        /// <summary>
+        /// Adds a pdf filepath/filename
+        /// </summary>
+        /// <param name="filename"></param>
+        public void AddFile(string filename)
+        {
+            Documents.Add(filename);
+        }
+
+        /// <summary>
+        /// Merges pdf files
+        /// </summary>
+        /// <param name="outputFile">optionally set outputfile here</param>
+        public void MergeFiles(string outputFile = null)
+        {
+            if (outputFile != null)
+                OutputFile = outputFile;
+            if (Documents?.Count < 2)
+                throw new ArgumentException("Two or more files are required to merge!");
+            if (OutputFile == null)
+                throw new ArgumentNullException("Output file not set!");
+            if ((OutputFile ?? string.Empty) == string.Empty)
+                throw new ArgumentException("Output file not set!");
+
+            var outDocument = new PdfDocument();
+
+            using (PdfDocument outPdf = new PdfDocument())
+            {
+                foreach (var document in Documents)
+                {
+                    PdfDocument importPdf = PdfReader.Open(document.Replace(@"\", "/"), PdfDocumentOpenMode.Import);
+                    CopyPages(importPdf, outPdf);
+                }
+                var outputLocation = OutputFile.Replace(@"\", "/");
+                outPdf.Save(outputLocation);
+            }
+        }
+
+        private void CopyPages(PdfDocument from, PdfDocument to)
+        {
+            for (int i = 0; i < from.PageCount; i++)
+            {
+                to.AddPage(from.Pages[i]);
+            }
+        }
+    }
+}