C# 检查和修改表达式<func<t, bool> >? </func<t,>

Question

My generic repository method is declared like this:

public virtual async Task<List<T>> GetMany(Expression<Func<T, bool>> filterExpression)

I would like to investigate that filterExpression and check if T is of a certain type AND if so, check if there's a filter for a property called ShardKey . Also, if the type checks out but the filter is not present, I would like to add it.

How can I achieve this?

Answer 1

Using an ExpressionVisitor you can search the Expression tree to determine if a reference to the desired property is made. If you wanted to be more sure, you could test to see if it is in a certain type of logical test as well.

If you don't find the desired reference, you can add a parent test to the filterExpression .

Here is an example, assuming ShardKey is an int :

public virtual async Task<List<T>> GetMany<T>(Expression<Func<T, bool>> filterExpression) {
    if (typeof(T) == typeof(CertainType) && !filterExpression.ReferencesMember<CertainType>("ShardKey")) {
        var skTest = Expression.Equal(Expression.PropertyOrField(filterExpression.Parameters[0], "ShardKey"), Expression.Constant(7));
        filterExpression = Expression.Lambda<Func<T,bool>>(Expression.AndAlso(skTest, filterExpression.Body), filterExpression.Parameters);
    }

Here is the ExpressionVisitor implementation:

public static class ExpressionExt {
    public static bool ReferencesMember<T>(this Expression ex, string memberName) {
        var mi = typeof(T).GetMember(memberName);
        if (mi.Length != 1)
            throw new ArgumentException($"{memberName} ambiguous or not found for {typeof(T).Name}");

        var mf = new MemberFinder(mi[0]);
        mf.Visit(ex);
        return mf.Found;
    }

    class MemberFinder : ExpressionVisitor {
        MemberInfo mi;
        public bool Found = false;

        public MemberFinder(MemberInfo _mi) => mi = _mi;

        [return: NotNullIfNotNull("node")]
        public override Expression Visit(Expression node) {
            if (Found)
                return node;
            else
                return base.Visit(node);
        }

        protected override Expression VisitMember(MemberExpression me) {
            if (me.Member == mi)
                Found = true;

            return me;
        }
    }
}