JsonConverter for {ReadOnly}Memory doesn't deserialize nulls

[stephentoub]

Given that this code runs successfully without exception or assert:

using System.Diagnostics;
using System.Text.Json;

ReadOnlyMemory<float> rom;

rom = default;
Debug.Assert(rom.IsEmpty);

rom = null;
Debug.Assert(rom.IsEmpty);

rom = (ReadOnlyMemory<float>)null;
Debug.Assert(rom.IsEmpty);

rom = ((float[])null!).AsMemory();
Debug.Assert(rom.IsEmpty);

rom = JsonSerializer.Deserialize<float[]>("null");
Debug.Assert(rom.IsEmpty);

it breaks my expectations that this:

rom = JsonSerializer.Deserialize<ReadOnlyMemory<float>>("null"); // boom

throws an exception:

Unhandled exception. System.Text.Json.JsonException: The JSON value could not be converted to System.ReadOnlyMemory`1[System.Single]. Path: $ | LineNumber: 0 | BytePositionInLine: 4.
   at System.Text.Json.ThrowHelper.ThrowJsonException_DeserializeUnableToConvertValue(Type propertyType)
   at System.Text.Json.Serialization.JsonConverter`1.TryRead(Utf8JsonReader& reader, Type typeToConvert, JsonSerializerOptions options, ReadStack& state, T& value, Boolean& isPopulatedValue)
   at System.Text.Json.Serialization.JsonConverter`1.ReadCore(Utf8JsonReader& reader, JsonSerializerOptions options, ReadStack& state)
   at System.Text.Json.Serialization.Metadata.JsonTypeInfo`1.Deserialize(Utf8JsonReader& reader, ReadStack& state)
   at System.Text.Json.JsonSerializer.ReadFromSpan[TValue](ReadOnlySpan`1 utf8Json, JsonTypeInfo`1 jsonTypeInfo, Nullable`1 actualByteCount)
   at System.Text.Json.JsonSerializer.ReadFromSpan[TValue](ReadOnlySpan`1 json, JsonTypeInfo`1 jsonTypeInfo)
   at System.Text.Json.JsonSerializer.Deserialize[TValue](String json, JsonSerializerOptions options)

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Issue Details

---

Given that this code runs successfully without exception or assert:

using System.Diagnostics;
using System.Text.Json;

ReadOnlyMemory<float> rom;

rom = default;
Debug.Assert(rom.IsEmpty);

rom = null;
Debug.Assert(rom.IsEmpty);

rom = (ReadOnlyMemory<float>)null;
Debug.Assert(rom.IsEmpty);

rom = ((float[])null!).AsMemory();
Debug.Assert(rom.IsEmpty);

rom = JsonSerializer.Deserialize<float[]>("null");
Debug.Assert(rom.IsEmpty);

it breaks my expectations that this:

rom = JsonSerializer.Deserialize<ReadOnlyMemory<float>>("null"); // boom

throws an exception:

Unhandled exception. System.Text.Json.JsonException: The JSON value could not be converted to System.ReadOnlyMemory`1[System.Single]. Path: $ | LineNumber: 0 | BytePositionInLine: 4.
   at System.Text.Json.ThrowHelper.ThrowJsonException_DeserializeUnableToConvertValue(Type propertyType)
   at System.Text.Json.Serialization.JsonConverter`1.TryRead(Utf8JsonReader& reader, Type typeToConvert, JsonSerializerOptions options, ReadStack& state, T& value, Boolean& isPopulatedValue)
   at System.Text.Json.Serialization.JsonConverter`1.ReadCore(Utf8JsonReader& reader, JsonSerializerOptions options, ReadStack& state)
   at System.Text.Json.Serialization.Metadata.JsonTypeInfo`1.Deserialize(Utf8JsonReader& reader, ReadStack& state)
   at System.Text.Json.JsonSerializer.ReadFromSpan[TValue](ReadOnlySpan`1 utf8Json, JsonTypeInfo`1 jsonTypeInfo, Nullable`1 actualByteCount)
   at System.Text.Json.JsonSerializer.ReadFromSpan[TValue](ReadOnlySpan`1 json, JsonTypeInfo`1 jsonTypeInfo)
   at System.Text.Json.JsonSerializer.Deserialize[TValue](String json, JsonSerializerOptions options)

Author:	stephentoub
Assignees:	-
Labels:	area-System.Text.Json
Milestone:	-

[eiriktsarpalis]

cc @jozkee, do you think we should try change this to support null JSON tokens?

[jozkee]

I believe so, yes. Just FWIW, readonly struct ImmutableArray<T> also throws this exception, but I believe ImmutableArray<T> doesn't have any bonds with null as opposed to this one. I'm not aware of any more "struct collections" to use as a reference for the semantic.