DataTable的Select方法的filterExpression参数以及DataTable的Column的Expression的说明

本文详细介绍了.NET Framework中DataColumn.Expression属性的功能与用法,包括如何使用表达式创建计算列和聚合列,支持的操作符、函数及语法规范等内容。

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DataColumn.Expression Property

.NET Framework 4
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Gets or sets the expression used to filter rows, calculate the values in a column, or create an aggregate column.

Namespace:   System.Data
Assembly:  System.Data (in System.Data.dll)
public string Expression { get; set; }
Property Value
Type:  System.String
An expression to calculate the value of a column, or create an aggregate column. The return type of an expression is determined by the  DataType of the column.
Exception Condition
ArgumentException

The AutoIncrement or Unique property is set to true.

FormatException

When you are using the CONVERT function, the expression evaluates to a string, but the string does not contain a representation that can be converted to the type parameter.

InvalidCastException

When you are using the CONVERT function, the requested cast is not possible. See the Conversion function in the following section for detailed information about possible casts.

ArgumentOutOfRangeException

When you use the SUBSTRING function, the start argument is out of range.

-Or-

When you use the SUBSTRING function, the length argument is out of range.

Exception

When you use the LEN function or the TRIM function, the expression does not evaluate to a string. This includes expressions that evaluate to Char.

One use of the Expression property is to create calculated columns. For example, to calculate a tax value, the unit price is multiplied by a tax rate of a specific region. Because tax rates vary from region to region, it would be impossible to put a single tax rate in a column; instead, the value is calculated using the Expression property, as shown in the Visual Basic code in the following section:

DataSet1.Tables("Products").Columns("tax").Expression = "UnitPrice * 0.086"

A second use is to create an aggregate column. Similar to a calculated value, an aggregate performs an operation based on the complete set of rows in the DataTable. A simple example is to count the number of rows returned in the set. This is the method you would use to count the number of transactions completed by a particular salesperson, as shown in this Visual Basic code:

 DataSet1.Tables("Orders").Columns("OrderCount").Expression = "Count(OrderID)"

Expression Syntax

When you create an expression, use the ColumnName property to refer to columns. For example, if the ColumnName for one column is "UnitPrice", and another "Quantity", the expression would be as follows:

"UnitPrice * Quantity"

NoteNote

If a column is used in an expression, then the expression is said to have a dependency on that column. If a dependent column is renamed or removed, no exception is thrown. An exception will be thrown when the now-broken expression column is accessed.

When you create an expression for a filter, enclose strings with single quotation marks:

"LastName = 'Jones'"

If a column name contains any non-alphanumeric characters or starts with a digit or matches (case-insensitively) any of the following reserved words, it requires special handling, as described in the following paragraphs.

And

Between

Child

False

In

Is

Like

Not

Null

Or

Parent

True

If a column name satisfies one of the above conditions, it must be wrapped in either square brackets or the "`" (grave accent) quotes. For example, to use a column named "Column#" in an expression, you would write either "[Column#]":

Total * [Column#]

or "`Column#`":

Total * `Column#`

If the column name is enclosed in square brackets then any ']' and '\' characters (but not any other characters) in it must be escaped by prepending them with the backslash ("\") character. If the column name is enclosed in grave accent characters then it must not contain any grave accent characters in it. For example, a column named "Column[]\" would be written:

Total * [Column[\]\\]

or

Total * `Column[]\`

User-Defined Values

User-defined values may be used within expressions to be compared with column values. String values should be enclosed within single quotation marks (and each single quotation character in a string value has to be escaped by prepending it with another single quotation character). Date values should be enclosed within pound signs (#) or single quotes (') based on the data provider. Decimals and scientific notation are permissible for numeric values. For example:

"FirstName = 'John'"

"Price <= 50.00"

"Birthdate < #1/31/82#"

For columns that contain enumeration values, cast the value to an integer data type. For example:

"EnumColumn = 5"

Parsing Literal Expressions

All literal expressions must be expressed in the invariant culture locale. When DataSet parses and converts literal expressions, it always uses the invariant culture, not the current culture.

String literals are identified when there are single quotes surrounding the value. For example:

'John'

Boolean literals are true and false; they are not quoted in expressions.

Integer literals [+-]?[0-9]+ are treated as System.Int32System.Int64 or System.DoubleSystem.Double can lose precision depending on how large the number is. For example, if the number in the literal is 2147483650, DataSet will first attempt to parse the number as an Int32. This will not succeed because the number is too large. In this case DataSet will parse the number as an Int64, which will succeed. If the literal was a number larger than the maximum value of an Int64, DataSet will parse the literal using Double.

Real literals using scientific notation, such as 4.42372E-30, are parsed using System.Double.

Real literals without scientific notation, but with a decimal point, are treated as System.Decimal. If the number exceeds the maximum or minimum values supported bySystem.Decimal, then it is parsed as a System.Double. For example:

142526.144524 will be converted to a Decimal.

345262.78036719560925667 will be treated as a Double.

Operators

Concatenation is allowed using Boolean AND, OR, and NOT operators. You can use parentheses to group clauses and force precedence. The AND operator has precedence over other operators. For example:

(LastName = 'Smith' OR LastName = 'Jones') AND FirstName = 'John'

When you create comparison expressions, the following operators are allowed:

<

>

<=

>=

<>

=

IN

LIKE

The following arithmetic operators are also supported in expressions:

+ (addition)

- (subtraction)

* (multiplication)

/ (division)

% (modulus)

String Operators

To concatenate a string, use the + character. The value of the CaseSensitive property of the DataSet class determines whether string comparisons are case-sensitive. However, you can override that value with the CaseSensitive property of the DataTable class.

Wildcard Characters

Both the * and % can be used interchangeably for wildcard characters in a LIKE comparison. If the string in a LIKE clause contains a * or %, those characters should be enclosed in brackets ([]). If a bracket is in the clause, each bracket character should be enclosed in brackets (for example [[] or []]). A wildcard is allowed at the start and end of a pattern, or at the end of a pattern, or at the start of a pattern. For example:

"ItemName LIKE '*product*'"

"ItemName LIKE '*product'"

"ItemName LIKE 'product*'"

Wildcard characters are not allowed in the middle of a string. For example, 'te*xt' is not allowed.

Parent/Child Relation Referencing

A parent table may be referenced in an expression by prepending the column name with Parent. For example, the Parent.Price references the parent table's column named Price.

When a child has more than one parent row, use Parent(RelationName).ColumnName. For example, the Parent(RelationName).Price references the parent table’s column named Price via the relation.

A column in a child table may be referenced in an expression by prepending the column name with Child. However, because child relationships may return multiple rows, you must include the reference to the child column in an aggregate function. For example, Sum(Child.Price) would return the sum of the column named Price in the child table.

If a table has more than one child, the syntax is: Child(RelationName). For example, if a table has two child tables named Customers and Orders, and the DataRelationobject is named Customers2Orders, the reference would be as follows:

Avg(Child(Customers2Orders).Quantity)

Aggregates

The following aggregate types are supported:

Sum (Sum)

Avg (Average)

Min (Minimum)

Max (Maximum)

Count (Count)

StDev (Statistical standard deviation)

Var (Statistical variance).

Aggregates are ordinarily performed along relationships. Create an aggregate expression by using one of the functions listed earlier and a child table column as detailed in Parent/Child Relation Referencing that was discussed earlier. For example:

Avg(Child.Price)

Avg(Child(Orders2Details).Price)

An aggregate can also be performed on a single table. For example, to create a summary of figures in a column named "Price":

Sum(Price)

NoteNote

If you use a single table to create an aggregate, there would be no group-by functionality. Instead, all rows would display the same value in the column.

If a table has no rows, the aggregate functions will return null.

Data types can always be determined by examining the DataType property of a column. You can also convert data types using the Convert function, shown in the following section.

Functions

The following functions are also supported:

CONVERT

Description

Converts particular expression to a specified .NET Framework Type.

Syntax

Convert(expressiontype)

Arguments

expression -- The expression to convert.

type -- The .NET Framework type to which the value will be converted.

Example: myDataColumn.Expression="Convert(total, 'System.Int32')"

All conversions are valid with the following exceptions: Boolean can be coerced to and from ByteSByteInt16Int32Int64UInt16UInt32UInt64String and itself only. Char can be coerced to and from Int32UInt32String, and itself only. DateTime can be coerced to and from String and itself only. TimeSpan can be coerced to and from String and itself only.

LEN

Description

Gets the length of a string

Syntax

LEN(expression)

Arguments

expression -- The string to be evaluated.

Example: myDataColumn.Expression="Len(ItemName)"

ISNULL

Description

Checks an expression and either returns the checked expression or a replacement value.

Syntax

ISNULL(expressionreplacementvalue)

Arguments

expression -- The expression to check.

replacementvalue -- If expression is nullreplacementvalue is returned.

Example: myDataColumn.Expression="IsNull(price, -1)"

IIF

Description

Gets one of two values depending on the result of a logical expression.

Syntax

IIF(exprtruepartfalsepart)

Arguments

expr -- The expression to evaluate.

truepart -- The value to return if the expression is true.

falsepart -- The value to return if the expression is false.

Example: myDataColumn.Expression = "IIF(total>1000, 'expensive', 'dear')

TRIM

Description

Removes all leading and trailing blank characters like \r, \n, \t, ' '

Syntax

TRIM(expression)

Arguments

expression -- The expression to trim.

SUBSTRING

Description

Gets a sub-string of a specified length, starting at a specified point in the string.

Syntax

SUBSTRING(expression, start, length)

Arguments

expression -- The source string for the substring.

start -- Integer that specifies where the substring starts.

length -- Integer that specifies the length of the substring.

Example: myDataColumn.Expression = "SUBSTRING(phone, 7, 8)"

NoteNote

You can reset the Expression property by assigning it a null value or empty string. If a default value is set on the expression column, all previously filled rows are assigned the default value after the Expression property is reset.

The following example creates three columns in a DataTable. The second and third columns contain expressions; the second calculates tax using a variable tax rate, and the third adds the result of the calculation to the value of the first column. The resulting table is displayed in a DataGrid control.

C#
VB
private void CalcColumns()
{
    DataTable table = new DataTable ();

    // Create the first column.
    DataColumn priceColumn = new DataColumn();
    priceColumn.DataType = System.Type.GetType("System.Decimal");
    priceColumn.ColumnName = "price";
    priceColumn.DefaultValue = 50;

    // Create the second, calculated, column.
    DataColumn taxColumn = new DataColumn();
    taxColumn.DataType = System.Type.GetType("System.Decimal");
    taxColumn.ColumnName = "tax";
    taxColumn.Expression = "price * 0.0862";

    // Create third column.
    DataColumn totalColumn = new DataColumn();
    totalColumn.DataType = System.Type.GetType("System.Decimal");
    totalColumn.ColumnName = "total";
    totalColumn.Expression = "price + tax";

    // Add columns to DataTable.
    table.Columns.Add(priceColumn);
    table.Columns.Add(taxColumn);
    table.Columns.Add(totalColumn);

    DataRow row = table.NewRow();
    table.Rows.Add(row);
    DataView view = new DataView(table);
    dataGrid1.DataSource = view;
}


.NET Framework
Supported in: 4, 3.5, 3.0, 2.0, 1.1, 1.0
.NET Framework Client Profile
Supported in: 4, 3.5 SP1
<think>好的,我现在需要解决用户提供的这段C#代码报错的问题。错误是System.Data.SyntaxErrorException,提示“構文エラー : '=' 演算子の前にオペランドがありません。”,也就是在等号运算符前缺少操作数。 首先,我需要分析代码中的问题所在。用户代码中,filterExpression的构建是关键部分,尤其是这一行: $$string filterExpression = $"{columnName} =='{selectID}'";$$ 错误信息指出在等号前缺少操作数,可能和这里的字符串拼接有关。根据DataColumn.Expression的语法规则,正确的比较运算符应该是单等号"=",而不是双等号"=="。在C#中,双等号用于逻辑比较,但DataTable的表达式语法可能遵循不同的规则,比如类似SQL的语法,使用单等号。 接下来检查字符串拼接是否正确。用户使用了$"{columnName} =='{selectID}'",如果columnName的值是"案件ID",那么生成的表达式应该是"案件ID =='某个值'",但这里应该用单等号。所以正确的应该是"案件ID = '某个值'"。将双等号改为单等号应该可以解决这个问题。 另外,还需要确认columnName是否存在空格或特殊字符,这时候需要用方括号括起来,比如"[案件ID]"。不过原代码中的columnName赋值是字符串"案件ID",假设这个列名确实存在且正确,但如果有空格的话,必须用方括号包裹。所以可能需要修改columnName的赋值,或者直接在filterExpression中添加方括号。 综合以上分析,可能的修改步骤是: 1. 将双等号"=="改为单等号"="。 2. 确保列名正确,如果有空格则用方括号括起来。 修改后的代码应该是: $$string filterExpression = $"{columnName} = '{selectID}'";$$ 或者如果列名包含空格: $$string filterExpression = $"[{columnName}] = '{selectID}'";$$ 还需要检查selectID是否存在可能导致SQL注入或格式问题的字符,不过在这个上下文中,用户可能已经控制了输入,所以暂时不考虑。此外,检查dataGridView2是否存在数据绑定问题,但错误信息更可能指向filterExpression的构建。 测试一下修改后的代码是否解决了语法错误,并且是否能正确过滤数据。如果仍然报错,可能需要进一步检查列名或数据内容,比如是否有空值或类型不匹配的问题,但当前错误更可能是语法问题。</think>让我们逐步分析并解决该错误: **错误原因分析:** 1. **运算符使用错误**:DataTable.Select()使用的表达式语法要求使用`=`而不是`==`进行比较操作 2. **列名格式问题**:当列名包含特殊字符或空格时,需要使用方括号包裹 **修改后的正确代码:** ```csharp if (e.RowIndex < 0) return; string selectID = Convert.ToString(dataGridView1.Rows[e.RowIndex].Cells[0].Value); textBox3.Text = selectID; // 修改点1:为列名添加方括号 string columnName = "[案件ID]"; if (CheckIfStringExistsInColumn(dataGridView2, 0, selectID)) { // 修改点2:将 == 改为 = string filterExpression = $"{columnName} = '{selectID}'"; DataRow[] filteredRows = NPC.allData2.Select(filterExpression); DataTable resultTable = NPC.allData2.Clone(); foreach (DataRow row in filteredRows) { resultTable.ImportRow(row); } dataGridView2.DataSource = resultTable; } ``` **改进建议:** 1. **参数化处理**(防SQL注入): ```csharp string filterExpression = $"{columnName} = @ID"; DataRow[] filteredRows = NPC.allData2.Select(filterExpression, new object[] { selectID }); ``` 2. **空值处理**: ```csharp string selectID = dataGridView1.Rows[e.RowIndex].Cells[0].Value?.ToString() ?? ""; ``` 3. **异常处理**: ```csharp try { DataRow[] filteredRows = NPC.allData2.Select(filterExpression); // ...后续处理... } catch (SyntaxErrorException ex) { MessageBox.Show($"过滤条件错误: {ex.Message}"); } ``` **关键修改点说明:** - `==` → `=`:DataTable的表达式语法使用单个等号进行值比较 - `[案件ID]`:方括号包裹列名是良好的编程习惯,能避免列名含空格等特殊字符时的解析错误 - 使用参数化查询可以防止特殊字符导致的错误和SQL注入风险 **常见问题延伸:** 1. 如果`selectID`可能包含单引号,需要进行转义: ```csharp string safeID = selectID.Replace("'", "''"); filterExpression = $"{columnName} = '{safeID}'"; ``` 2. 数值型比较不需要引号: ```csharp // 如果案件ID是数字类型 filterExpression = $"{columnName} = {selectID}"; ```
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