How to Combine Two Expression Expressions Using Logical Operators?

Two expressions of combination (expression

)

In the field of data operation, you may need to use the two Expression

type expressions with logical operators (such as AND, OR, or NOT). Challenge

The following code fragments illustrate the challenges you may face:

Solution

To use the two expressions of logical operators, you can choose the following methods:

<code>Expression<func bool="">> expr1;
Expression<func bool="">> expr2;

...

// 这不会按预期工作
Expression<func bool="">> andExpression = expr1 AND expr2;</func></func></func></code>

Method 1: The same parameterexpression

If you use the same parameterexpression in EXPR1 and Expr2, you can simply combine their text and use the original parameter to create a new Lambda expression.

This method is also applicable to negating a single expression:

<code>var body = Expression.AndAlso(expr1.Body, expr2.Body);
var lambda = Expression.Lambda<func bool="">(body, expr1.Parameters[0]);</func></code>

Method 2: Different parameterexpressions

If two expressions use different ParameterexPression instances, according to your Linq provider, you can use Invoke to combine them:

<code>static Expression<func bool=""> Not<t>(
    this Expression<func bool=""> expr)
{
    return Expression.Lambda<func bool="">(
        Expression.Not(expr.Body), expr.Parameters[0]);
}</func></func></t></func></code>

This method is suitable for AND and OR operations.

Method 3: General Version

<code>static Expression<func bool=""> AndAlso<t>(
    this Expression<func bool=""> left,
    Expression<func bool=""> right)
{
    var param = Expression.Parameter(typeof(T), "x");
    var body = Expression.AndAlso(
            Expression.Invoke(left, param),
            Expression.Invoke(right, param)
        );
    var lambda = Expression.Lambda<func bool="">(body, param);
    return lambda;
}</func></func></func></t></func></code>

This version will automatically detect whether the two expressions use the same parameters and adjust the expression accordingly:

Use ExpressionVisitor

For .NET 4.0 and higher versions, you can use the ExpressionVisitor class to create an EF security expression. This method allows you to replace the original parameter with the number of new parameters without explicit conversion:

<code>static Expression<func bool=""> AndAlso<t>(
    this Expression<func bool=""> expr1,
    Expression<func bool=""> expr2)
{
    // 检测相同的参数
    ParameterExpression param = expr1.Parameters[0];
    if (ReferenceEquals(param, expr2.Parameters[0]))
    {
        return Expression.Lambda<func bool="">(
            Expression.AndAlso(expr1.Body, expr2.Body), param);
    }
    // 否则，使用 Invoke 来组合
    return Expression.Lambda<func bool="">(
        Expression.AndAlso(
            expr1.Body,
            Expression.Invoke(expr2, param)), param);
}</func></func></func></func></t></func></code>

By using these methods, you can seamlessly combine various logical expressions and perform complex query operations on your data.

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