C# reflection get method with no parameters

Question

I have this code:

MethodInfo method = typeof(AppDbContext)
     .GetMethod(nameof(AppDbContext.Set), BindingFlags.Public | BindingFlags.Instance);

There are two methods named Set in DbContext, Set<>() and Set<>(string name) the problem is, there is ambiguity with this code. So how do I get the method with no parameters?

Answer 1

For such reflection manipulations, usually I'm using simple helper class, which extracts MethodInfo from LambdaExpression.

Your code can be simplified via this helper in the following manner:

MethodInfo method = MemberHelper.MethodOfGeneric((AppDbContext ctx) => ctx.Set<object>());

And helper class implementation:

public static class MemberHelper
{
    public static MethodInfo MethodOf<TResult>(Expression<Func<TResult>>                         methodCall) => GetMethodInfo(methodCall);
    public static MethodInfo MethodOf<T1, TResult>(Expression<Func<T1, TResult>>                 methodCall) => GetMethodInfo(methodCall);
    public static MethodInfo MethodOf<T1, T2, TResult>(Expression<Func<T1, T2, TResult>>         methodCall) => GetMethodInfo(methodCall);
    public static MethodInfo MethodOf<T1, T2, T3, TResult>(Expression<Func<T1, T2, T3, TResult>> methodCall) => GetMethodInfo(methodCall);

    public static MethodInfo MethodOfGeneric<TResult>(Expression<Func<TResult>>                         methodCall) => GetMethodInfo(methodCall).GetGenericMethodDefinition();
    public static MethodInfo MethodOfGeneric<T1, TResult>(Expression<Func<T1, TResult>>                 methodCall) => GetMethodInfo(methodCall).GetGenericMethodDefinition();
    public static MethodInfo MethodOfGeneric<T1, T2, TResult>(Expression<Func<T1, T2, TResult>>         methodCall) => GetMethodInfo(methodCall).GetGenericMethodDefinition();
    public static MethodInfo MethodOfGeneric<T1, T2, T3, TResult>(Expression<Func<T1, T2, T3, TResult>> methodCall) => GetMethodInfo(methodCall).GetGenericMethodDefinition();

    static MethodInfo GetMethodInfo(LambdaExpression lambda)
    {
        if (GetMemberInfo(lambda) is not MethodInfo methodInfo)
            throw new InvalidOperationException("Expression must be a method call expression.");

        return methodInfo;
    }

    static MemberInfo GetMemberInfo(LambdaExpression func)
    {
        return GetMemberInfo(func.Body);
    }

    static MemberInfo GetMemberInfo(Expression expr)
    {
        if (expr is MemberExpression memberExpr)
            return memberExpr.Member;

        if (expr is MethodCallExpression methodExpr)
            return methodExpr.Method;

        throw new InvalidOperationException("Expression must be a member expression or a method call expression.");
    }
}

It can be extended to get constructors, properties, etc.