using System.Linq.Expressions;

public static TDelegate CreateAccessor<TDelegate>(string memberName) where TDelegate : Delegate
{
    var invokeMethod = typeof(TDelegate).GetMethod("Invoke");
    if (invokeMethod == null)
        throw new InvalidOperationException($"{typeof(TDelegate)} signature could not be determined.");

    var delegateParameters = invokeMethod.GetParameters();
    if (delegateParameters.Length != 1)
        throw new InvalidOperationException("Delegate must have a single parameter.");

    var paramType = delegateParameters[0].ParameterType;

    var objParam = Expression.Parameter(paramType, "obj");
    var memberExpr = Expression.PropertyOrField(objParam, memberName);
    Expression returnExpr = memberExpr;
    if (invokeMethod.ReturnType != memberExpr.Type)
        returnExpr = Expression.ConvertChecked(memberExpr, invokeMethod.ReturnType);

    var lambda = Expression.Lambda<TDelegate>(returnExpr, $"Access{paramType.Name}_{memberName}", new [] { objParam });
    return lambda.Compile();
}

ref struct Foo
{
    private string A;
    private int B;

    public Foo(string a, int b)
    {
        A = a;
        B = b;
    }
}

delegate object FooAccessor(Foo foo);
// or
delegate T FooAccessor<T>(Foo foo);

var foo = new Foo("abc", 123);
var a = CreateAccessor<FooAccessor<string>>("A")(foo);
var b = CreateAccessor<FooAccessor<int>>("B")(foo);