# DataValidation
```csharp
public sealed class DataValidation
```
Namespace `SharpCell` ยท package `SharpCell`
A data validation rule: what the cells of a range accept and the messages Excel shows for them. Set properties in an initializer, or start from [DataValidation.List](SharpCell.DataValidation.md#list-string), [DataValidation.ListFrom](SharpCell.DataValidation.md#listfrom-string) or [DataValidation.Custom](SharpCell.DataValidation.md#custom-string). SharpCell keeps the rule with the cells and saves it; it does not check values.
## Constructors
### DataValidation()
```csharp
public DataValidation()
```
A rule allowing any value; set properties in an initializer.
### DataValidation(DataValidation)
```csharp
public DataValidation(DataValidation source)
```
A copy of another rule, so a factory's rule can be changed in an initializer: `new DataValidation(DataValidation.List("Yes", "No")) { ErrorMessage = "Choose Yes or No." }`.
**Parameters**
- `source`: The rule to copy.
**Exceptions**
- `ArgumentNullException`: The source is null.
## Properties
### AllowBlank
```csharp
public bool AllowBlank { get; init; }
```
Whether an empty cell is accepted; true unless set.
### ErrorMessage
```csharp
public string? ErrorMessage { get; init; }
```
The error message, at most 255 characters, or null for Excel's own.
**Exceptions**
- `ArgumentException`: The message is longer than 255 characters.
### ErrorStyle
```csharp
public ValidationErrorStyle ErrorStyle { get; init; }
```
What Excel does with a value that breaks the rule; [ValidationErrorStyle.Stop](SharpCell.ValidationErrorStyle.md#stop) unless set.
### ErrorTitle
```csharp
public string? ErrorTitle { get; init; }
```
The title of the error message, at most 32 characters, or null for Excel's own.
**Exceptions**
- `ArgumentException`: The title is longer than 32 characters.
### Formula1
```csharp
public string? Formula1 { get; init; }
```
The first formula, with a leading `=` (added if missing) as [Cell.Formula](SharpCell.Cell.md#formula) takes it, or null: the low end or the value compared with, the list (`="Yes,No"` or `=$H$1:$H$5`) or the custom condition (`=A1>0`). Relative references are relative to the first cell of the rule's first range. It is parsed when the rule is added to a sheet.
### Formula2
```csharp
public string? Formula2 { get; init; }
```
The second formula, the high end for [ValidationOperator.Between](SharpCell.ValidationOperator.md#between) and [ValidationOperator.NotBetween](SharpCell.ValidationOperator.md#notbetween), written like [DataValidation.Formula1](SharpCell.DataValidation.md#formula1); null for none.
### InputMessage
```csharp
public string? InputMessage { get; init; }
```
The input message, at most 255 characters, or null for none.
**Exceptions**
- `ArgumentException`: The message is longer than 255 characters.
### InputTitle
```csharp
public string? InputTitle { get; init; }
```
The title of the input message, at most 32 characters, or null for none.
**Exceptions**
- `ArgumentException`: The title is longer than 32 characters.
### Operator
```csharp
public ValidationOperator Operator { get; init; }
```
How a value is compared with [DataValidation.Formula1](SharpCell.DataValidation.md#formula1) and [DataValidation.Formula2](SharpCell.DataValidation.md#formula2); [ValidationOperator.Between](SharpCell.ValidationOperator.md#between) unless set. Lists and custom rules ignore it.
### ShowDropDown
```csharp
public bool ShowDropDown { get; init; }
```
Whether Excel shows the drop-down of a list rule; true unless set.
### ShowErrorMessage
```csharp
public bool ShowErrorMessage { get; init; }
```
Whether Excel shows the error message when a value breaks the rule; true unless set.
### ShowInputMessage
```csharp
public bool ShowInputMessage { get; init; }
```
Whether Excel shows the input message when a cell of the rule is selected; true unless set.
### Type
```csharp
public ValidationType Type { get; init; }
```
What the cells may hold; [ValidationType.Any](SharpCell.ValidationType.md#any) unless set.
## Methods
### Custom(string)
```csharp
public static DataValidation Custom(string formula)
```
A rule accepting a value when the formula is true, such as `Custom("=A1>0")`.
**Parameters**
- `formula`: The condition, relative to the first cell of the rule's first range.
**Exceptions**
- `ArgumentException`: The formula is null or empty.
### Equals(DataValidation)
```csharp
public bool Equals(DataValidation? other)
```
Whether the other rule has the same settings; texts are compared ordinally.
### Equals(object)
```csharp
public override bool Equals(object? obj)
```
Whether `obj` is a rule with the same settings.
### GetHashCode()
```csharp
public override int GetHashCode()
```
A hash code consistent with [DataValidation.Equals](SharpCell.DataValidation.md#equals-datavalidation).
### List(string[])
```csharp
public static DataValidation List(params string[] items)
```
A drop-down list of the given items, kept in the rule as one quoted literal: `List("Yes", "No")` has the formula `="Yes,No"`.
**Parameters**
- `items`: The items, in order. An item cannot hold a comma, which separates them.
**Exceptions**
- `ArgumentNullException`: The items or one of them is null.
- `ArgumentException`: There are no items, an item holds a comma, or the items with their commas are longer than 255 characters, Excel's limit.
### ListFrom(string)
```csharp
public static DataValidation ListFrom(string range)
```
A drop-down list of the values of a range, such as `ListFrom("$H$1:$H$5")` or a defined name.
**Parameters**
- `range`: The range or name, with or without a leading `=`.
**Exceptions**
- `ArgumentException`: The range is null or empty.
### ToString()
```csharp
public override string ToString()
```
The type, the operator of a comparing type, and the formulas, such as `WholeNumber Between =1 =10`.