Ensure non-nullability, validate encoding, and clean up nullable qualifiers

* `SerializerBase.SerializationContext`: Ensure it is initialized and non-null by design
* `ServiceProvider.GetService(...)` returns a non-null instance or throws an exception if the service is unavailable.
* `EncodingProvider.DecoderDelegate` throws for invalid encoding argument
* `EncoderDelegate GetEncoderFor` throws for invalid encoding argument
* Remove unneccessary conditional access qualifier according to nullable reference types' annotations

Author: axunonb

Ical.Net.Tests/EncodingProviderTests.cs

@@ -5,6 +5,7 @@
 
 #nullable enable
 using System;
+using System.Runtime.Serialization;
 using Ical.Net.Serialization;
 using NUnit.Framework;
 using EncodingProvider = Ical.Net.Serialization.EncodingProvider;
@@ -33,7 +34,7 @@ public void Encode_ShouldBeNull_WhenInvalidEncodingIsProvided()
         const string encoding = "Invalid-Encoding";
         var data = "Hello"u8.ToArray();
 
-        Assert.That(GetEncodingProvider().Encode(encoding, data), Is.Null);
+        Assert.That(() => GetEncodingProvider().Encode(encoding, data), Throws.TypeOf<SerializationException>());
     }
 
     [Test]
@@ -53,7 +54,7 @@ public void Decode_ShouldBeNull_WhenInvalidEncodingIsProvided()
         const string encoding = "Invalid-Encoding";
         const string data = "Hello";
 
-        Assert.That(GetEncodingProvider().DecodeString(encoding, data), Is.Null);
+        Assert.That(() => GetEncodingProvider().DecodeString(encoding, data), Throws.TypeOf<SerializationException>());
     }
 
     [Test]

Ical.Net/Calendar.cs

@@ -121,7 +121,7 @@ public override int GetHashCode()
     {
         unchecked
         {
-            var hashCode = Name?.GetHashCode() ?? 0;
+            var hashCode = Name.GetHashCode();
             hashCode = (hashCode * 397) ^ CollectionHelpers.GetHashCode(UniqueComponents);
             hashCode = (hashCode * 397) ^ CollectionHelpers.GetHashCode(Events);
             hashCode = (hashCode * 397) ^ CollectionHelpers.GetHashCode(Todos);
@@ -290,8 +290,6 @@ public virtual void MergeWith(IMergeable obj)
             return;
         }
 
-        Name ??= c.Name;
-
         Method = c.Method;
         Version = c.Version;
         ProductId = c.ProductId;

Ical.Net/CalendarComponents/Alarm.cs

@@ -172,7 +172,7 @@ private void AddRepeatedItems(List<AlarmOccurrence> occurrences)
         for (var i = 0; i < len; i++)
         {
             var ao = occurrences[i];
-            if (ao?.DateTime == null || ao.Component == null)
+            if (ao.DateTime == null || ao.Component == null)
             {
                 continue;
             }

Ical.Net/CalendarComponents/Todo.cs

@@ -85,7 +85,7 @@ public virtual int PercentComplete
     public virtual IList<string> Resources
     {
         get => Properties.GetMany<string>("RESOURCES");
-        set => Properties.Set("RESOURCES", value ?? new List<string>());
+        set => Properties.Set("RESOURCES", value);
     }
 
     /// <summary>

Ical.Net/CalendarComponents/VTimeZone.cs

@@ -383,7 +383,7 @@ public override int GetHashCode()
     {
         unchecked
         {
-            var hashCode = Name?.GetHashCode() ?? 0;
+            var hashCode = Name.GetHashCode();
             hashCode = (hashCode * 397) ^ (TzId?.GetHashCode() ?? 0);
             hashCode = (hashCode * 397) ^ (Url?.GetHashCode() ?? 0);
             return hashCode;

Ical.Net/Collections/Proxies/GroupedValueListProxy.cs

@@ -64,7 +64,7 @@ private void IterateValues(Func<IValueObject<TOriginalValue>, int, int, bool> ac
         foreach (var obj in _realObject)
         {
             // Get the number of items of the target value i this object
-            var count = obj.Values?.OfType<TNewValue>().Count() ?? 0;
+            var count = obj.Values.OfType<TNewValue>().Count();
 
             // Perform some action on this item
             if (!action(obj, i, count))

Ical.Net/DataTypes/Attachment.cs

@@ -113,8 +113,8 @@ public override int GetHashCode()
         unchecked
         {
             var hashCode = Uri?.GetHashCode() ?? 0;
-            hashCode = (hashCode * 397) ^ (CollectionHelpers.GetHashCode(Data));
-            hashCode = (hashCode * 397) ^ (ValueEncoding?.GetHashCode() ?? 0);
+            hashCode = (hashCode * 397) ^ (Data != null ? CollectionHelpers.GetHashCode(Data) : 0);
+            hashCode = (hashCode * 397) ^ (ValueEncoding.GetHashCode());
             return hashCode;
         }
     }

Ical.Net/DataTypes/Occurrence.cs

@@ -41,7 +41,7 @@ public override int GetHashCode()
     {
         unchecked
         {
-            return ((Period?.GetHashCode() ?? 0) * 397) ^ (Source?.GetHashCode() ?? 0);
+            return ((Period.GetHashCode()) * 397) ^ (Source.GetHashCode());
         }
     }
 

Ical.Net/DataTypes/PeriodList.cs

@@ -125,7 +125,7 @@ public void Insert(int index, Period item)
     }
 
     /// <inheritdoc/>
-    public void RemoveAt(int index) => Periods?.RemoveAt(index);
+    public void RemoveAt(int index) => Periods.RemoveAt(index);
 
     /// <summary>
     /// Adds a <see cref="Period"/> to the list if it does not already exist.<br/>
@@ -159,7 +159,7 @@ public void Add(CalDateTime dt)
     public bool Contains(Period item) => Periods.Contains(item);
 
     /// <inheritdoc/>
-    public void CopyTo(Period[] array, int arrayIndex) => Periods?.CopyTo(array, arrayIndex);
+    public void CopyTo(Period[] array, int arrayIndex) => Periods.CopyTo(array, arrayIndex);
 
     /// <inheritdoc/>
     public IEnumerator<Period> GetEnumerator() => Periods.GetEnumerator();

Ical.Net/DataTypes/Trigger.cs

@@ -119,7 +119,7 @@ public override int GetHashCode()
         {
             var hashCode = _mDateTime?.GetHashCode() ?? 0;
             hashCode = (hashCode * 397) ^ _mDuration.GetHashCode();
-            hashCode = (hashCode * 397) ^ _mRelated?.GetHashCode() ?? 0;
+            hashCode = (hashCode * 397) ^ _mRelated.GetHashCode();
             return hashCode;
         }
     }