ktsu-dev · matt-edmondson · Sep 13, 2026 · Sep 13, 2026
diff --git a/CLAUDE.md b/CLAUDE.md
@@ -23,11 +23,13 @@ dotnet run -c Release --project PreciseNumber.Benchmarks -- --filter '*' --job s
 
 ### Core Types
 
-- **PreciseNumber** (`PreciseNumber/PreciseNumber.cs`): The main numeric type implementing `INumber<PreciseNumber>`. Stores values using:
+- **PreciseNumber** (`PreciseNumber/PreciseNumber.cs`): The main numeric type, a `readonly partial record struct` implementing `INumber<PreciseNumber>`. Its `default` is `Zero`, which `PreciseNumberValueTypeTests` pins. Stores values using:
   - `Significand`: A `BigInteger` containing all significant digits
   - `Exponent`: An `int` determining the decimal place
   - `SignificantDigits`: Count of significant digits
 
+- **Generic conversions** (`PreciseNumber/PreciseNumber.Conversions.cs`): The `TryConvertFrom*` and `TryConvertTo*` members behind `CreateChecked`, `CreateSaturating`, and `CreateTruncating`, for every built-in numeric type and `BigInteger`. `To<T>()` uses them too
+
 - **PreciseNumberExtensions** (`PreciseNumber/PreciseNumberExtensions.cs`): Extension methods providing `ToPreciseNumber<T>()` for converting any `INumber<T>` to PreciseNumber
 
 ### Key Design Patterns
@@ -37,10 +39,12 @@ dotnet run -c Release --project PreciseNumber.Benchmarks -- --filter '*' --job s
 - `Divide` is exact when the quotient terminates, and otherwise rounds to a precision that never falls below the wider operand or `MinimumDivisionPrecision`. `Exp` and non-integer `Pow` still route through `double`
 - The `sanitize` constructor parameter controls whether trailing zeros are removed (default: true)
 - Constants (`Zero`, `One`, `Pi`, `E`, `Tau`) are pre-computed static instances
+- As a value type it can't be null or inherited. Don't add null checks for `PreciseNumber` parameters, and don't reintroduce `protected` members
+- Conversions to integer types go through `BigInteger`, so range checks, clamping, and wrapping follow its conventions. Conversions to `double`, `float`, `Half`, and `decimal` render `significand E exponent` and parse it, because the runtime parsers round correctly, with Clinger's fast path for small values. NaN and infinity coming in follow `BigInteger` too
 
 ### Test Structure
 
-Tests use MSTest framework in `PreciseNumber.Test/PreciseNumberTests.cs`. The test project targets only .NET 10.0 while the main library multi-targets net7.0, net8.0, net9.0, and net10.0.
+Tests use MSTest. `PreciseNumber.Test/PreciseNumberTests.cs` covers arithmetic, parsing, and formatting, `PreciseNumberConversionTests.cs` covers generic math conversion in every mode, and `PreciseNumberValueTypeTests.cs` pins `default` as zero and asserts that small-value addition, subtraction, multiplication, and comparison allocate nothing. The test project targets only .NET 10.0 while the main library multi-targets net7.0, net8.0, net9.0, and net10.0.
 
 ### Benchmarks
 
@@ -52,9 +56,10 @@ Most classes are parameterised by `Digits` (8, 30, 200). That axis is the point:
 `BigInteger`, so anything that touches them one at a time looks fine at 8 digits and collapses at
 200. Read results across the `Digits` column, not down one value of it.
 
-Allocation is reported alongside time and matters just as much — every operation returns a new
-instance, so avoiding an intermediate shows up in `Allocated` before it shows up in `Mean`.
-Comparisons should allocate nothing at all.
+Allocation is reported alongside time and matters just as much. The number is a value type, so the
+only allocations are `BigInteger` digit arrays, and avoiding an intermediate shows up in `Allocated`
+before it shows up in `Mean`. Comparisons, and addition, subtraction, and multiplication of
+significands that fit in an `int`, should allocate nothing at all.
 
 Run the relevant benchmarks before and after any change to the library's internals. See
 `PreciseNumber.Benchmarks/README.md` for details.
diff --git a/PreciseNumber.Test/PreciseNumberConversionTests.cs b/PreciseNumber.Test/PreciseNumberConversionTests.cs
@@ -0,0 +1,298 @@
+// Copyright (c) 2023-2026 ktsu-dev contributors
+
+namespace ktsu.PreciseNumber.Test;
+
+using System.Globalization;
+using System.Numerics;
+
+/// <summary>
+/// Covers conversion through generic math, which is how code written against <see cref="INumber{TSelf}"/>
+/// reaches <see cref="PreciseNumber"/>.
+/// </summary>
+[TestClass]
+public class PreciseNumberConversionTests
+{
+	private static PreciseNumber P(string text) => PreciseNumber.Parse(text, CultureInfo.InvariantCulture);
+
+	private static TTo Checked<TTo, TFrom>(TFrom value)
+		where TTo : INumberBase<TTo>
+		where TFrom : INumberBase<TFrom>
+		=> TTo.CreateChecked(value);
+
+	private static TTo Saturating<TTo, TFrom>(TFrom value)
+		where TTo : INumberBase<TTo>
+		where TFrom : INumberBase<TFrom>
+		=> TTo.CreateSaturating(value);
+
+	private static TTo Truncating<TTo, TFrom>(TFrom value)
+		where TTo : INumberBase<TTo>
+		where TFrom : INumberBase<TFrom>
+		=> TTo.CreateTruncating(value);
+
+	/// <summary>The shape of a unit conversion in generic quantity code.</summary>
+	private static T ToBase<T>(T value, double factor)
+		where T : INumber<T>
+		=> value * T.CreateChecked(factor);
+
+	private static void AssertFromInAllModes<TFrom>(TFrom value, string expected)
+		where TFrom : INumberBase<TFrom>
+	{
+		PreciseNumber expectedNumber = P(expected);
+		Assert.AreEqual(expectedNumber, Checked<PreciseNumber, TFrom>(value), $"Checked from {typeof(TFrom).Name}");
+		Assert.AreEqual(expectedNumber, Saturating<PreciseNumber, TFrom>(value), $"Saturating from {typeof(TFrom).Name}");
+		Assert.AreEqual(expectedNumber, Truncating<PreciseNumber, TFrom>(value), $"Truncating from {typeof(TFrom).Name}");
+	}
+
+	private static void AssertToInAllModes<TTo>(string value, TTo expected)
+		where TTo : INumberBase<TTo>
+	{
+		PreciseNumber number = P(value);
+		Assert.AreEqual(expected, Checked<TTo, PreciseNumber>(number), $"Checked to {typeof(TTo).Name}");
+		Assert.AreEqual(expected, Saturating<TTo, PreciseNumber>(number), $"Saturating to {typeof(TTo).Name}");
+		Assert.AreEqual(expected, Truncating<TTo, PreciseNumber>(number), $"Truncating to {typeof(TTo).Name}");
+	}
+
+	[TestMethod]
+	public void FromEveryIntegerTypeIsExact()
+	{
+		AssertFromInAllModes(sbyte.MinValue, "-128");
+		AssertFromInAllModes(byte.MaxValue, "255");
+		AssertFromInAllModes(short.MinValue, "-32768");
+		AssertFromInAllModes(ushort.MaxValue, "65535");
+		AssertFromInAllModes(int.MinValue, "-2147483648");
+		AssertFromInAllModes(uint.MaxValue, "4294967295");
+		AssertFromInAllModes(long.MinValue, "-9223372036854775808");
+		AssertFromInAllModes(ulong.MaxValue, "18446744073709551615");
+		AssertFromInAllModes(Int128.MinValue, "-170141183460469231731687303715884105728");
+		AssertFromInAllModes(UInt128.MaxValue, "340282366920938463463374607431768211455");
+		AssertFromInAllModes((nint)(-42), "-42");
+		AssertFromInAllModes((nuint)42, "42");
+		AssertFromInAllModes('A', "65");
+		AssertFromInAllModes(BigInteger.Pow(10, 60) + 1, "1" + new string('0', 59) + "1");
+		AssertFromInAllModes(1000, "1000");
+	}
+
+	[TestMethod]
+	public void FromDecimalIsExact()
+	{
+		AssertFromInAllModes(1234.5678m, "1234.5678");
+		AssertFromInAllModes(decimal.MaxValue, "79228162514264337593543950335");
+		AssertFromInAllModes(decimal.MinValue, "-79228162514264337593543950335");
+		AssertFromInAllModes(0.0000000000000000000000000001m, "0.0000000000000000000000000001");
+	}
+
+	[TestMethod]
+	public void FromBinaryFloatingPointUsesDecimalText()
+	{
+		AssertFromInAllModes(0.3048, "0.3048");
+		AssertFromInAllModes(-1.5e-10, "-0.00000000015");
+		AssertFromInAllModes(1.25f, "1.25");
+		AssertFromInAllModes((Half)1.5, "1.5");
+		AssertFromInAllModes(0.0, "0");
+	}
+
+	[TestMethod]
+	public void FromPreciseNumberIsIdentity()
+	{
+		AssertFromInAllModes(P("-12.5"), "-12.5");
+	}
+
+	[TestMethod]
+	public void GenericUnitConversionIsExact()
+	{
+		Assert.AreEqual(P("0.3048"), ToBase(PreciseNumber.One, 0.3048));
+		Assert.AreEqual(P("3.048"), ToBase(10.ToPreciseNumber(), 0.3048));
+	}
+
+	[TestMethod]
+	public void FromNaNMatchesBigInteger()
+	{
+		// BigInteger is the other built-in numeric type with no NaN and no bound, so it sets the convention.
+		Assert.ThrowsExactly<OverflowException>(() => Checked<BigInteger, double>(double.NaN));
+		Assert.AreEqual(BigInteger.Zero, Saturating<BigInteger, double>(double.NaN));
+		Assert.AreEqual(BigInteger.Zero, Truncating<BigInteger, double>(double.NaN));
+
+		Assert.ThrowsExactly<OverflowException>(() => Checked<PreciseNumber, double>(double.NaN));
+		Assert.AreEqual(PreciseNumber.Zero, Saturating<PreciseNumber, double>(double.NaN));
+		Assert.AreEqual(PreciseNumber.Zero, Truncating<PreciseNumber, float>(float.NaN));
+		Assert.AreEqual(PreciseNumber.Zero, Saturating<PreciseNumber, Half>(Half.NaN));
+	}
+
+	[TestMethod]
+	public void FromInfinityMatchesBigInteger()
+	{
+		Assert.ThrowsExactly<OverflowException>(() => Checked<BigInteger, double>(double.PositiveInfinity));
+		Assert.ThrowsExactly<OverflowException>(() => Saturating<BigInteger, double>(double.PositiveInfinity));
+		Assert.ThrowsExactly<OverflowException>(() => Truncating<BigInteger, double>(double.NegativeInfinity));
+
+		Assert.ThrowsExactly<OverflowException>(() => Checked<PreciseNumber, double>(double.PositiveInfinity));
+		Assert.ThrowsExactly<OverflowException>(() => Saturating<PreciseNumber, double>(double.PositiveInfinity));
+		Assert.ThrowsExactly<OverflowException>(() => Truncating<PreciseNumber, float>(float.NegativeInfinity));
+	}
+
+	[TestMethod]
+	public void TryConvertReportsUnsupportedTypes()
+	{
+		Assert.IsFalse(PreciseNumber.TryConvertFromChecked(new Complex(1, 0), out PreciseNumber _), "Complex is not a supported source");
+		Assert.IsFalse(PreciseNumber.TryConvertToChecked(PreciseNumber.One, out Complex _), "Complex is not a supported destination");
+	}
+
+	[TestMethod]
+	public void ToIntegerTruncatesTowardZero()
+	{
+		AssertToInAllModes("12.9", 12);
+		AssertToInAllModes("-12.9", -12);
+		AssertToInAllModes("0.999", 0L);
+		AssertToInAllModes("-0.5", (short)0);
+		AssertToInAllModes("12345e3", 12345000);
+		AssertToInAllModes("65.7", 'A');
+		AssertToInAllModes("-170141183460469231731687303715884105728", Int128.MinValue);
+		AssertToInAllModes("340282366920938463463374607431768211455", UInt128.MaxValue);
+		AssertToInAllModes("-42.1", (nint)(-42));
+		AssertToInAllModes("42.1", (nuint)42);
+		AssertToInAllModes("255", byte.MaxValue);
+		AssertToInAllModes("-128", sbyte.MinValue);
+		AssertToInAllModes("65535", ushort.MaxValue);
+		AssertToInAllModes("4294967295", uint.MaxValue);
+		AssertToInAllModes("18446744073709551615.5", ulong.MaxValue);
+	}
+
+	[TestMethod]
+	public void ToIntegerOutOfRange()
+	{
+		PreciseNumber tooLargeForByte = P("300");
+		Assert.ThrowsExactly<OverflowException>(() => Checked<byte, PreciseNumber>(tooLargeForByte));
+		Assert.AreEqual(byte.MaxValue, Saturating<byte, PreciseNumber>(tooLargeForByte));
+		Assert.AreEqual((byte)44, Truncating<byte, PreciseNumber>(tooLargeForByte));
+
+		PreciseNumber negative = P("-1");
+		Assert.ThrowsExactly<OverflowException>(() => Checked<uint, PreciseNumber>(negative));
+		Assert.AreEqual(uint.MinValue, Saturating<uint, PreciseNumber>(negative));
+		Assert.AreEqual(uint.MaxValue, Truncating<uint, PreciseNumber>(negative));
+
+		PreciseNumber pastInt = P("2147483648");
+		Assert.ThrowsExactly<OverflowException>(() => Checked<int, PreciseNumber>(pastInt));
+		Assert.AreEqual(int.MaxValue, Saturating<int, PreciseNumber>(pastInt));
+		Assert.AreEqual(int.MinValue, Truncating<int, PreciseNumber>(pastInt));
+
+		Assert.ThrowsExactly<OverflowException>(() => Checked<char, PreciseNumber>(P("70000")));
+	}
+
+	[TestMethod]
+	public void ToIntegerFromHugeValuesMatchesBigInteger()
+	{
+		foreach (string text in new[] { "7e5000", "-7e5000", "123456789e50", "-98765e45", "3e39", "1e38" })
+		{
+			PreciseNumber number = P(text);
+			BigInteger integer = number.To<BigInteger>();
+
+			Assert.AreEqual(Truncating<int, BigInteger>(integer), Truncating<int, PreciseNumber>(number), $"int truncating {text}");
+			Assert.AreEqual(Truncating<ulong, BigInteger>(integer), Truncating<ulong, PreciseNumber>(number), $"ulong truncating {text}");
+			Assert.AreEqual(Truncating<Int128, BigInteger>(integer), Truncating<Int128, PreciseNumber>(number), $"Int128 truncating {text}");
+			Assert.AreEqual(Truncating<UInt128, BigInteger>(integer), Truncating<UInt128, PreciseNumber>(number), $"UInt128 truncating {text}");
+			Assert.AreEqual(Saturating<long, BigInteger>(integer), Saturating<long, PreciseNumber>(number), $"long saturating {text}");
+			Assert.AreEqual(Saturating<UInt128, BigInteger>(integer), Saturating<UInt128, PreciseNumber>(number), $"UInt128 saturating {text}");
+		}
+
+		Assert.ThrowsExactly<OverflowException>(() => Checked<Int128, PreciseNumber>(P("7e5000")));
+	}
+
+	[TestMethod]
+	public void ToBigIntegerTruncatesTowardZero()
+	{
+		AssertToInAllModes("12345e5", BigInteger.Parse("1234500000", CultureInfo.InvariantCulture));
+		AssertToInAllModes("-12.9", new BigInteger(-12));
+		AssertToInAllModes("0.0001", BigInteger.Zero);
+	}
+
+	[TestMethod]
+	public void ToDoubleIsCorrectlyRounded()
+	{
+		string[] texts =
+		[
+			"0.1",
+			"0.3048",
+			"-123.456",
+			"3.14159265358979323846264338327950288419716939937510582097494",
+			"9007199254740993",
+			"9007199254740995",
+			"2.2250738585072011e-308",
+			"4.9406564584124654e-324",
+			"1.7976931348623157e308",
+			"123456789012345678901234567890e-45",
+			"0.000000000000000000000000000000000000000001",
+		];
+
+		foreach (string text in texts)
+		{
+			double expected = double.Parse(text, NumberStyles.Float, CultureInfo.InvariantCulture);
+			AssertToInAllModes(text, expected);
+		}
+	}
+
+	[TestMethod]
+	public void ToDoubleOverflowsToInfinity()
+	{
+		AssertToInAllModes("1e400", double.PositiveInfinity);
+		AssertToInAllModes("-1e400", double.NegativeInfinity);
+		AssertToInAllModes("1e-400", 0.0);
+	}
+
+	[TestMethod]
+	public void ToSingleAndHalfAreCorrectlyRounded()
+	{
+		foreach (string text in new[] { "0.1", "16777217", "3.4028235e38", "1.401298464324817e-45", "2.718281828459045235360287471352662497757" })
+		{
+			AssertToInAllModes(text, float.Parse(text, NumberStyles.Float, CultureInfo.InvariantCulture));
+		}
+
+		foreach (string text in new[] { "0.1", "65504", "65520", "1.5", "-3.14159" })
+		{
+			AssertToInAllModes(text, Half.Parse(text, NumberStyles.Float, CultureInfo.InvariantCulture));
+		}
+	}
+
+	[TestMethod]
+	public void ToDecimalRoundsExcessDigits()
+	{
+		AssertToInAllModes("1234.5678", 1234.5678m);
+		AssertToInAllModes("79228162514264337593543950335", decimal.MaxValue);
+
+		const string longFraction = "0.12345678901234567890123456789012345678901234";
+		AssertToInAllModes(longFraction, decimal.Parse(longFraction, NumberStyles.Float, CultureInfo.InvariantCulture));
+	}
+
+	[TestMethod]
+	public void ToDecimalOutOfRange()
+	{
+		foreach (string text in new[] { "1e30", "79228162514264337593543950336" })
+		{
+			PreciseNumber number = P(text);
+			Assert.ThrowsExactly<OverflowException>(() => Checked<decimal, PreciseNumber>(number), text);
+			Assert.AreEqual(decimal.MaxValue, Saturating<decimal, PreciseNumber>(number), text);
+			Assert.AreEqual(decimal.MaxValue, Truncating<decimal, PreciseNumber>(number), text);
+			Assert.AreEqual(decimal.MinValue, Saturating<decimal, PreciseNumber>(-number), text);
+		}
+
+		// BigInteger clamps rather than wraps when truncating to decimal, and so does PreciseNumber.
+		Assert.AreEqual(decimal.MaxValue, Truncating<decimal, BigInteger>(BigInteger.Pow(10, 30)));
+	}
+
+	[TestMethod]
+	public void BuiltInCreateCheckedReachesPreciseNumber()
+	{
+		Assert.AreEqual(1.5, double.CreateChecked(P("1.5")));
+		Assert.AreEqual(12, int.CreateChecked(P("12.75")));
+		Assert.AreEqual(1234.5678m, decimal.CreateSaturating(P("1234.5678")));
+		Assert.AreEqual(BigInteger.One, BigInteger.CreateTruncating(P("1.9")));
+	}
+
+	[TestMethod]
+	public void ToUsesTheSameConversions()
+	{
+		Assert.AreEqual(12, P("12.9").To<int>());
+		Assert.AreEqual(
+			double.Parse("3.14159265358979323846264338327950288419716939937510582097494", NumberStyles.Float, CultureInfo.InvariantCulture),
+			P("3.14159265358979323846264338327950288419716939937510582097494").To<double>());
+	}
+}