diff --git a/NHSE.Core/Structures/Building/BuildingType.cs b/NHSE.Core/Structures/Building/BuildingType.cs
index c274145..8c0ef12 100644
--- a/NHSE.Core/Structures/Building/BuildingType.cs
+++ b/NHSE.Core/Structures/Building/BuildingType.cs
@@ -3,9 +3,7 @@
     /// <summary>
     /// Building model &amp; interior that is loaded when the <see cref="Building"/> is entered.
     /// </summary>
-#pragma warning disable CA1027 // Mark enums with FlagsAttribute
     public enum BuildingType : ushort
-#pragma warning restore CA1027 // Mark enums with FlagsAttribute
     {
         None = 0,
         PlayerHouse1 = 1,
diff --git a/NHSE.Core/Structures/Item/ItemMenuIconType.cs b/NHSE.Core/Structures/Item/ItemMenuIconType.cs
index 22e166f..7363049 100644
--- a/NHSE.Core/Structures/Item/ItemMenuIconType.cs
+++ b/NHSE.Core/Structures/Item/ItemMenuIconType.cs
@@ -3,9 +3,7 @@ namespace NHSE.Core
     /// <summary>
     /// Menu icons (image shown in the inventory) of Items
     /// </summary>
-#pragma warning disable CA1027 // Mark enums with FlagsAttribute
     public enum ItemMenuIconType : ushort
-#pragma warning restore CA1027 // Mark enums with FlagsAttribute
     {
         Akoyagai,
         Amaebi,
diff --git a/NHSE.Core/Structures/Item/ItemSizeType.cs b/NHSE.Core/Structures/Item/ItemSizeType.cs
index c76b237..682a608 100644
--- a/NHSE.Core/Structures/Item/ItemSizeType.cs
+++ b/NHSE.Core/Structures/Item/ItemSizeType.cs
@@ -3,9 +3,7 @@
     /// <summary>
     /// Sizes of Items
     /// </summary>
-#pragma warning disable CA1027 // Mark enums with FlagsAttribute
     public enum ItemSizeType : byte
-#pragma warning restore CA1027 // Mark enums with FlagsAttribute
     {
         S_0_5x1_0_Wall,
         S_1_0x0_5,
diff --git a/NHSE.Parsing/Properties/Resources.Designer.cs b/NHSE.Parsing/Properties/Resources.Designer.cs
index 57c6fc1..14cec6a 100644
--- a/NHSE.Parsing/Properties/Resources.Designer.cs
+++ b/NHSE.Parsing/Properties/Resources.Designer.cs
@@ -8,6 +8,7 @@
 // </auto-generated>
 //------------------------------------------------------------------------------
 
+[assembly: System.Resources.NeutralResourcesLanguage("en")]
 namespace NHSE.Parsing.Properties {
     using System;
     
diff --git a/NHSE.Sprites/NHSE.Sprites.csproj b/NHSE.Sprites/NHSE.Sprites.csproj
index 1834f11..2a80a89 100644
--- a/NHSE.Sprites/NHSE.Sprites.csproj
+++ b/NHSE.Sprites/NHSE.Sprites.csproj
@@ -1,7 +1,7 @@
 <Project Sdk="Microsoft.NET.Sdk">
 
   <PropertyGroup>
-    <TargetFrameworks>net46;netcoreapp3.1</TargetFrameworks>
+    <TargetFrameworks>net46;net5.0</TargetFrameworks>
     <LangVersion>8</LangVersion>
     <Nullable>enable</Nullable>
   </PropertyGroup>
@@ -10,8 +10,8 @@
     <Folder Include="Resources\MenuIcon\" />
     <Folder Include="Resources\Villagers\" />
   </ItemGroup>
-  
-  <ItemGroup Condition="'$(TargetFramework)' == 'netcoreapp3.1'">
+
+  <ItemGroup Condition="$(TargetFramework.StartsWith('net5'))">
     <PackageReference Include="System.Drawing.Common" Version="4.7.0" />
     <PackageReference Include="System.Resources.Extensions" Version="4.7.0" />
   </ItemGroup>
diff --git a/NHSE.Sprites/Properties/Resources.Designer.cs b/NHSE.Sprites/Properties/Resources.Designer.cs
index bb9e3b3..07bfec5 100644
--- a/NHSE.Sprites/Properties/Resources.Designer.cs
+++ b/NHSE.Sprites/Properties/Resources.Designer.cs
@@ -8,6 +8,7 @@
 // </auto-generated>
 //------------------------------------------------------------------------------
 
+[assembly: System.Resources.NeutralResourcesLanguage("en")]
 namespace NHSE.Sprites.Properties {
     using System;
     
diff --git a/NHSE.Tests/NHSE.Tests.csproj b/NHSE.Tests/NHSE.Tests.csproj
index c3353a6..ef943ce 100644
--- a/NHSE.Tests/NHSE.Tests.csproj
+++ b/NHSE.Tests/NHSE.Tests.csproj
@@ -1,7 +1,7 @@
 <Project Sdk="Microsoft.NET.Sdk">
 
   <PropertyGroup>
-    <TargetFramework>netcoreapp3.1</TargetFramework>
+    <TargetFramework>net5.0</TargetFramework>
 
     <IsPackable>false</IsPackable>
   </PropertyGroup>
diff --git a/NHSE.Tests/Properties/Resources.Designer.cs b/NHSE.Tests/Properties/Resources.Designer.cs
index 599a499..2e0880a 100644
--- a/NHSE.Tests/Properties/Resources.Designer.cs
+++ b/NHSE.Tests/Properties/Resources.Designer.cs
@@ -8,6 +8,7 @@
 // </auto-generated>
 //------------------------------------------------------------------------------
 
+[assembly: System.Resources.NeutralResourcesLanguage("en")]
 namespace NHSE.Tests.Properties {
     using System;
     
diff --git a/NHSE.WinForms/Controls/ItemGridEditor.cs b/NHSE.WinForms/Controls/ItemGridEditor.cs
index deb46e6..b77d663 100644
--- a/NHSE.WinForms/Controls/ItemGridEditor.cs
+++ b/NHSE.WinForms/Controls/ItemGridEditor.cs
@@ -53,7 +53,7 @@ private void InitializeSlots()
             ChangePage();
         }
 
-        private void Slot_MouseWheel(object sender, MouseEventArgs e)
+        private void Slot_MouseWheel(object? sender, MouseEventArgs e)
         {
             var delta = e.Delta < 0 ? 1 : -1; // scrolling down increases page #
             var newpage = Math.Min(PageCount - 1, Math.Max(0, Page + delta));
@@ -85,8 +85,10 @@ public void Slot_MouseLeave(object? sender, EventArgs e)
 
         public static string GetItemText(Item item) => GameInfo.Strings.GetItemName(item);
 
-        public void Slot_MouseClick(object sender, MouseEventArgs e)
+        public void Slot_MouseClick(object? sender, MouseEventArgs e)
         {
+            if (sender == null)
+                return;
             switch (ModifierKeys)
             {
                 case Keys.Control | Keys.Alt: ClickClone(sender, e); break;
@@ -171,7 +173,7 @@ private void SetItemSprite(Item item, PictureBox pb)
             pb.Image = ItemSprite.GetItemMarkup(item, font, dw, dh, backing);
         }
 
-        private int GetPageJump()
+        private static int GetPageJump()
         {
             return ModifierKeys switch
             {
diff --git a/NHSE.WinForms/NHSE.WinForms.csproj b/NHSE.WinForms/NHSE.WinForms.csproj
index 8d16b58..1eb9cdf 100644
--- a/NHSE.WinForms/NHSE.WinForms.csproj
+++ b/NHSE.WinForms/NHSE.WinForms.csproj
@@ -2,7 +2,7 @@
 
   <PropertyGroup>
     <OutputType>WinExe</OutputType>
-    <TargetFrameworks>net46;netcoreapp3.1</TargetFrameworks>
+    <TargetFrameworks>net46;net5.0-windows</TargetFrameworks>
     <UseWindowsForms>true</UseWindowsForms>
     <AssemblyName>NHSE</AssemblyName>
     <PackageId>NHSE</PackageId>
diff --git a/NHSE.WinForms/Properties/Resources.Designer.cs b/NHSE.WinForms/Properties/Resources.Designer.cs
index 19b8e6e..347df7f 100644
--- a/NHSE.WinForms/Properties/Resources.Designer.cs
+++ b/NHSE.WinForms/Properties/Resources.Designer.cs
@@ -8,6 +8,7 @@
 // </auto-generated>
 //------------------------------------------------------------------------------
 
+[assembly: System.Resources.NeutralResourcesLanguage("en")]
 namespace NHSE.WinForms.Properties {
     using System;
     
diff --git a/NHSE.WinForms/Subforms/Program/ImageFetcher.cs b/NHSE.WinForms/Subforms/Program/ImageFetcher.cs
index cc53fe4..e99a888 100644
--- a/NHSE.WinForms/Subforms/Program/ImageFetcher.cs
+++ b/NHSE.WinForms/Subforms/Program/ImageFetcher.cs
@@ -71,7 +71,7 @@ private void B_Download_Click(object sender, EventArgs e)
 
         private void ProgressChanged(object sender, DownloadProgressChangedEventArgs e) => PBar_MultiUse.Value = e.ProgressPercentage;
 
-        private void Completed(object sender, AsyncCompletedEventArgs e)
+        private void Completed(object? sender, AsyncCompletedEventArgs e)
         {
             if (e.Error != null)
             {
@@ -138,8 +138,13 @@ private async void CheckNetworkFileSizeAsync()
             {
                 using var webClient = new WebClient();
                 await webClient.OpenReadTaskAsync(new Uri(AllHosts[CB_HostSelect.SelectedIndex], UriKind.Absolute)).ConfigureAwait(false);
-
-                var totalSizeBytes = Convert.ToInt64(webClient.ResponseHeaders["Content-Length"]);
+                var hdr = webClient.ResponseHeaders?["Content-Length"];
+                if (hdr == null)
+                {
+                    L_FileSize.Text = "Failed.";
+                    return;
+                }
+                var totalSizeBytes = Convert.ToInt64(hdr);
                 var totalSizeMb = totalSizeBytes / 1e+6;
                 L_FileSize.Text = $"{totalSizeMb:0.##}MB";
             }