How to Combine Logical Expressions of Type `Expression` in LINQ?

In the Linq, combine the logical expression of the type

Expression<Func<bool>> LINQ often uses expression, the expression is a representation of Lambda expression. When processing expressions, two logical expressions are often required, such as AND, OR, or Not.

For example, consider the following two expressions:

We may want to combine these expressions to obtain the same type of new expression:

Expression<Func<bool>> expr1;
Expression<Func<bool>> expr2;

<合> The expression of the combination with the same parameters

// 示例用法（原样无效）
Expression<Func<bool>> andExpression = expr1 AND expr2;

If <<> and use the same parameters, the operation is very simple:

<合> The expression of different parameters with a combination expr1 expr2

However, if the expression uses different parameters, we must pay attention to how to combine them. Use <现> to achieve this:

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

<用> General method

The following code fragment provides a common method to determine the most suitable way to determine the combination expression: Invoke

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;
}

<注意> Other precautions

In .NET 4.0 and higher versions, you can use

Class to create EF security expressions:

static Expression<Func<bool>> AndAlso<T>(
    this Expression<Func<bool>> expr1,
    Expression<Func<bool>> expr2)
{
    ParameterExpression param = expr1.Parameters[0];
    if (ReferenceEquals(param, expr2.Parameters[0]))
    {
        return Expression.Lambda<Func<bool>>(
            Expression.AndAlso(expr1.Body, expr2.Body), param);
    }
    return Expression.Lambda<Func<bool>>(
        Expression.AndAlso(
            expr1.Body,
            Expression.Invoke(expr2, param)), param);
}

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