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# Id
The id of a task, link, resource, assignment or calendar. An `Id` holds either a number or a string. Create one with `Id.Num` or `Id.Str`, or assign a `string`, `double` or `int` wherever an `Id` is expected, and the implicit conversion builds it.
Two ids are equal when their [`Key()`]() values are equal, so `1` and `"1"` are the same id.
## Declaration
```csharp
public sealed class Id : IEquatable
```
`Id` is immutable and has no public constructor.
## Properties
| Property | Type | Description |
|---|---|---|
| `IsNum` | `bool` | `true` for a numeric id. |
| `IsStr` | `bool` | `true` for a string id. When it's `true`, the compiler treats `StrValue` as non-null. |
| `NumValue` | `double?` | The number of a numeric id; `null` for a string id. |
| `StrValue` | `string?` | The text of a string id; `null` for a numeric id. |
## Methods
### Key
```csharp
public string Key()
```
Returns the id as text. A string id is returned as is. A number is printed the way JavaScript's `String(n)` prints it:
| Number | Key |
|---|---|
| `1` | `"1"` |
| `1.5` | `"1.5"` |
| `0` and `-0` | `"0"` |
| `1e21` | `"1e+21"` |
| `1e-7` | `"1e-7"` |
| `double.NaN` | `"NaN"` |
Equality, `GetHashCode()` and `ToString()` all use this key.
**Errors:** throws `InvalidOperationException` when the id is an infinite number.
### Norm
```csharp
public string Norm()
```
Same as [`Key()`]().
### Equals
```csharp
public bool Equals(Id? other)
public override bool Equals(object? obj)
```
Returns `true` when both ids have the same key: `Id.Num(1).Equals(Id.Str("1"))` is `true`. String ids compare with ordinal, case-sensitive comparison. Two numeric ids compare as numbers, and every NaN equals every other NaN. `Equals(null)` returns `false`.
A number beyond `double` precision is stored rounded, so `Id.Num(9007199254740993)` equals `Id.Str("9007199254740992")`.
**Errors:** throws `InvalidOperationException` when one id is a string and the other is an infinite number.
### GetHashCode
```csharp
public override int GetHashCode()
```
Hash of the key, so ids that are equal hash the same and `Id` works as a `Dictionary` or `HashSet` key.
**Errors:** throws `InvalidOperationException` for an infinite number.
### ToString
```csharp
public override string ToString()
```
Returns the key.
## Static methods
### Num
```csharp
public static Id Num(double n)
```
Creates a numeric id.
### Str
```csharp
public static Id Str(string s)
```
Creates a string id. The empty string is a valid id.
**Errors:** throws `ArgumentNullException` when `s` is `null`.
## Operators
```csharp
public static bool operator ==(Id? a, Id? b)
public static bool operator !=(Id? a, Id? b)
public static implicit operator Id(string s)
public static implicit operator Id(double n)
public static implicit operator Id(int n)
```
`==` and `!=` compare by key, like [`Equals`](). Two `null` references are equal; `null` and an id aren't.
The implicit conversions call `Id.Str` or `Id.Num`. Converting a `null` string throws `ArgumentNullException`.
## Validation
An id in project data must be a finite number or a string. A NaN or infinite numeric id throws [`InputException`]() with `Path` `"id"` when the data is loaded.
## Example
```csharp
using Plen;
Id a = 1; // Id.Num(1)
Id b = "1"; // Id.Str("1")
Id c = 1.5;
Console.WriteLine(a == b); // True
Console.WriteLine(a.IsNum); // True
Console.WriteLine(b.StrValue); // 1
Console.WriteLine(c.Key()); // 1.5
var names = new Dictionary { [a] = "Design" };
Console.WriteLine(names["1"]); // Design
```
## Related
- [`TaskData`]() - a record keyed by `Id`
- [`ProjectData`]() - id uniqueness rules
- [Records, ids and dates]() - ids, dates and the other input types
- [Project data]() - describing a plan