Fix NullReferenceException in GetBuildVersion with missing version file

Fix #24

Author: AArnott

src/NerdBank.GitVersioning.Tests/BuildIntegrationTests.cs

@@ -68,6 +68,7 @@ public async Task GetBuildVersion_In_Git_But_Without_Commits()
         Repository.Init(this.RepoPath);
         var repo = new Repository(this.RepoPath); // do not assign Repo property to avoid commits being generated later
         this.WriteVersionFile("3.4");
+        Assumes.False(repo.Head.Commits.Any()); // verification that the test is doing what it claims
         var buildResult = await this.BuildAsync();
         Assert.Equal("3.4", buildResult.BuildVersion);
         Assert.Equal("3.4.0", buildResult.AssemblyInformationalVersion);
@@ -80,6 +81,18 @@ public async Task GetBuildVersion_In_Git_But_Head_Lacks_VersionFile()
         var repo = new Repository(this.RepoPath); // do not assign Repo property to avoid commits being generated later
         repo.Commit("empty", this.Signer, this.Signer, new CommitOptions { AllowEmptyCommit = true });
         this.WriteVersionFile("3.4");
+        Assumes.True(repo.Index[VersionFile.JsonFileName] == null);
+        var buildResult = await this.BuildAsync();
+        Assert.Equal("0.0.1." + repo.Head.Commits.First().GetIdAsVersion().Revision, buildResult.BuildVersion);
+        Assert.Equal("0.0.1+g" + repo.Head.Commits.First().Id.Sha.Substring(0, 10), buildResult.AssemblyInformationalVersion);
+    }
+
+    [Fact]
+    public async Task GetBuildVersion_In_Git_No_VersionFile_At_All()
+    {
+        Repository.Init(this.RepoPath);
+        var repo = new Repository(this.RepoPath); // do not assign Repo property to avoid commits being generated later
+        repo.Commit("empty", this.Signer, this.Signer, new CommitOptions { AllowEmptyCommit = true });
         var buildResult = await this.BuildAsync();
         Assert.Equal("0.0.1." + repo.Head.Commits.First().GetIdAsVersion().Revision, buildResult.BuildVersion);
         Assert.Equal("0.0.1+g" + repo.Head.Commits.First().Id.Sha.Substring(0, 10), buildResult.AssemblyInformationalVersion);

src/Nerdbank.GitVersioning.Tasks/GetBuildVersion.cs

@@ -100,15 +100,15 @@ public override bool Execute()
                         VersionFile.GetVersion(commit) ??
                         VersionFile.GetVersion(Environment.CurrentDirectory);
 
-                    this.PrereleaseVersion = versionOptions.Version.Prerelease;
+                    this.PrereleaseVersion = versionOptions?.Version.Prerelease ?? string.Empty;
 
                     var repoRoot = git?.Info?.WorkingDirectory;
                     var relativeRepoProjectDirectory = !string.IsNullOrWhiteSpace(repoRoot)
                         ? Environment.CurrentDirectory.Substring(repoRoot.Length).TrimStart(Path.DirectorySeparatorChar, Path.AltDirectorySeparatorChar)
                         : null;
 
                     // Override the typedVersion with the special build number and revision components, when available.
-                    typedVersion = commit?.GetIdAsVersion(relativeRepoProjectDirectory) ?? versionOptions.Version.Version;
+                    typedVersion = commit?.GetIdAsVersion(relativeRepoProjectDirectory) ?? versionOptions?.Version.Version;
                 }
 
                 typedVersion = typedVersion ?? new Version();
@@ -117,7 +117,7 @@ public override bool Execute()
                     : new Version(typedVersion.Major, typedVersion.Minor);
                 this.SimpleVersion = typedVersionWithoutRevision.ToString();
                 this.MajorMinorVersion = new Version(typedVersion.Major, typedVersion.Minor).ToString();
-                this.AssemblyVersion = versionOptions.AssemblyVersion?.ToString() ?? this.MajorMinorVersion;
+                this.AssemblyVersion = versionOptions?.AssemblyVersion?.ToString() ?? this.MajorMinorVersion;
                 this.BuildNumber = Math.Max(0, typedVersion.Build);
                 this.Version = typedVersion.ToString();
             }