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copied!<p>You can use the <a href="http://msdn.microsoft.com/en-us/library/system.linq.expressions.aspx" rel="nofollow"><code>System.Linq.Expressions</code> namespace</a> to build an expression by hand. For the two examples you've posted, the following should work:</p> <pre><code>public static Expression<Func<T, bool>> StringEquals<T>(Expression<Func<T, string>> leftOperand, Expression<Func<T, string>> rightOperand) { var p = leftOperand.Parameters[0]; var leftOperandBody = leftOperand.Body; var rightOperandBody = ReplacementVisitor.Transform(rightOperand, rightOperand.Parameters[0], p); var isNullOrEmptyMethod = typeof(string).GetMethod("IsNullOrEmpty"); var leftNullOrEmpty = Expression.Call(isNullOrEmptyMethod, leftOperandBody); var rightNullOrEmpty = Expression.Call(isNullOrEmptyMethod, rightOperandBody); var bothNullOrEmpty = Expression.AndAlso(leftNullOrEmpty, rightNullOrEmpty); var areEqual = Expression.Equal(leftOperandBody, rightOperandBody); var body = Expression.OrElse(bothNullOrEmpty, areEqual); return Expression.Lambda<Func<T, bool>>(body, p); } public static Expression<Func<T, bool>> DatesActive<T>(Expression<Func<T, DateTime>> startDate, Expression<Func<T, DateTime>> endDate) { var p = startDate.Parameters[0]; var startDateBody = startDate.Body; var endDateBody = ReplacementVisitor.Transform(endDate, endDate.Parameters[0], p); var nowProperty = typeof(DateTime).GetProperty("Now"); var nowValue = Expression.Property(null, nowProperty); var startValid = Expression.GreaterThanOrEqual(startDateBody, nowValue); var endValid = Expression.LessThanOrEqual(endDateBody, nowValue); var body = Expression.AndAlso(startValid, endValid); return Expression.Lambda<Func<T, bool>>(body, p); } internal sealed class ReplacementVisitor : ExpressionVisitor { private IList<ParameterExpression> SourceParameters { get; set; } private Expression Find { get; set; } private Expression Replace { get; set; } public static Expression Transform(LambdaExpression source, Expression find, Expression replace) { var visitor = new ReplacementVisitor { SourceParameters = source.Parameters, Find = find, Replace = replace, }; return visitor.Visit(source.Body); } private Expression ReplaceNode(Expression node) { return (node == Find) ? Replace : node; } protected override Expression VisitConstant(ConstantExpression node) { return ReplaceNode(node); } protected override Expression VisitBinary(BinaryExpression node) { var result = ReplaceNode(node); if (result == node) result = base.VisitBinary(node); return result; } protected override Expression VisitParameter(ParameterExpression node) { if (SourceParameters.Contains(node)) return ReplaceNode(node); return SourceParameters.FirstOrDefault(p => p.Name == node.Name) ?? node; } } </code></pre>

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