ReplaceFileContents.cs

using System;
using System.Collections.Generic;
using System.IO;
using System.Linq;
using System.Text;

using Microsoft.Build.Framework;
using Microsoft.Build.Utilities;

namespace Xamarin.Android.BuildTools.PrepTasks
{
	public class ReplaceFileContents : Task
	{
		[Required]
		public  ITaskItem   SourceFile          { get; set; }

		[Required]
		public  ITaskItem   DestinationFile     { get; set; }

		public  string[]    Replacements        { get; set; }

		public override bool Execute ()
		{
			Log.LogMessage (MessageImportance.Low, $"Task {nameof (ReplaceFileContents)}");
			Log.LogMessage (MessageImportance.Low, $"  {nameof (SourceFile)}: {SourceFile.ItemSpec}");
			Log.LogMessage (MessageImportance.Low, $"  {nameof (DestinationFile)}: {DestinationFile.ItemSpec}");
			Log.LogMessage (MessageImportance.Low, $"  {nameof (Replacements)}:");
			foreach (var replacement in Replacements) {
				Log.LogMessage (MessageImportance.Low, $"    {replacement}");
			}

			File.Delete (DestinationFile.ItemSpec);

			var r   = GetReplacementInfo (Replacements);
			using (var i    = File.OpenText (SourceFile.ItemSpec))
			using (var o    = File.CreateText (DestinationFile.ItemSpec)) {
				string line;
				while ((line = i.ReadLine ()) != null) {
					foreach (var e in r) {
						line = line.Replace (e.Key, e.Value);
					}
					o.WriteLine (line);
				}
			}

			return !Log.HasLoggedErrors;
		}

		static  readonly    char[]  Separator   = new [] { '=' };

		static Dictionary<string, string> GetReplacementInfo (string[] replacements)
		{
			var r   = new Dictionary<string, string> (replacements?.Length ?? 0);
			if (replacements == null)
				return r;
			foreach (var e in replacements) {
				if (string.IsNullOrEmpty (e))
					continue;
				var kvp = e.Split (Separator, 2, StringSplitOptions.RemoveEmptyEntries);
				r.Add (kvp [0], kvp.Length > 1 ? kvp [1] : "");
			}
			return r;
		}
	}
}